load_graph_from_csv(): Change default to directed=False

6 jobs for master
Status Job ID Name Coverage
  Test
canceled #3894
amd64
job_clang_amd64

canceled #3898
amd64
job_clang_amd64_nosh

canceled #3896
amd64
job_clang_py2_amd64

canceled #3893
amd64
job_gcc_amd64

canceled #3897
amd64
job_gcc_amd64_nosh

canceled #3895
amd64
job_gcc_py2_amd64