README.md: Update minimal compiler requirements

This fixes issue #585.
6 jobs for master in 412 minutes and 25 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
failed #4009
amd64
job_clang_amd64

06:39:20

failed #4013
amd64
job_clang_amd64_nosh

06:39:47

failed #4011
amd64
job_clang_py2_amd64

04:35:43

passed #4008
amd64
job_gcc_amd64

06:52:22

passed #4012
amd64
job_gcc_amd64_nosh

06:19:40

failed #4010
amd64
job_gcc_py2_amd64

04:46:05

 
Name Stage Failure
failed
job_clang_amd64_nosh Test
<string>:53: RuntimeWarning: More than 20 figures have been opened. Figures created through the pyplot interface (`matplotlib.pyplot.figure`) are retained until explicitly closed and may consume too much memory. (To control this warning, see the rcParam `figure.max_open_warning`).
<string>:1: RuntimeWarning: More than 20 figures have been opened. Figures created through the pyplot interface (`matplotlib.pyplot.figure`) are retained until explicitly closed and may consume too much memory. (To control this warning, see the rcParam `figure.max_open_warning`).
<string>:53: RuntimeWarning: More than 20 figures have been opened. Figures created through the pyplot interface (`matplotlib.pyplot.figure`) are retained until explicitly closed and may consume too much memory. (To control this warning, see the rcParam `figure.max_open_warning`).
/var/lib/gitlab-runner/builds/c098b1de/0/count0/graph-tool/doc/price.py:94: RuntimeWarning: More than 20 figures have been opened. Figures created through the pyplot interface (`matplotlib.pyplot.figure`) are retained until explicitly closed and may consume too much memory. (To control this warning, see the rcParam `figure.max_open_warning`).
figure(figsize=(6,4))
/var/lib/gitlab-runner/builds/c098b1de/0/count0/graph-tool/doc/price.py:104: UserWarning: tight_layout : falling back to Agg renderer
tight_layout()
<string>:53: RuntimeWarning: More than 20 figures have been opened. Figures created through the pyplot interface (`matplotlib.pyplot.figure`) are retained until explicitly closed and may consume too much memory. (To control this warning, see the rcParam `figure.max_open_warning`).
ERROR: Job failed: exit status 1
failed
job_clang_amd64 Test
 1178 tests
3 failures in tests
0 failures in setup code
0 failures in cleanup code
build finished with problems, 19 warnings.
<string>:53: RuntimeWarning: More than 20 figures have been opened. Figures created through the pyplot interface (`matplotlib.pyplot.figure`) are retained until explicitly closed and may consume too much memory. (To control this warning, see the rcParam `figure.max_open_warning`).
<string>:53: RuntimeWarning: More than 20 figures have been opened. Figures created through the pyplot interface (`matplotlib.pyplot.figure`) are retained until explicitly closed and may consume too much memory. (To control this warning, see the rcParam `figure.max_open_warning`).
<string>:53: RuntimeWarning: More than 20 figures have been opened. Figures created through the pyplot interface (`matplotlib.pyplot.figure`) are retained until explicitly closed and may consume too much memory. (To control this warning, see the rcParam `figure.max_open_warning`).
ERROR: Job failed: exit status 1
failed
job_gcc_py2_amd64 Test

Exception occurred:
File "/usr/lib/python2.7/site-packages/sphinx/ext/autodoc/__init__.py", line 82, in members_option
return [x.strip() for x in arg.split(',')]
AttributeError: 'bool' object has no attribute 'split'
The full traceback has been saved in /tmp/sphinx-err-7trLUa.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!
ERROR: Job failed: exit status 1
failed
job_clang_py2_amd64 Test

Exception occurred:
File "/usr/lib/python2.7/site-packages/sphinx/ext/autodoc/__init__.py", line 82, in members_option
return [x.strip() for x in arg.split(',')]
AttributeError: 'bool' object has no attribute 'split'
The full traceback has been saved in /tmp/sphinx-err-ahV_Fs.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!
ERROR: Job failed: exit status 1