numpy_bind.hh: Replace new with malloc

Numpy will use free() not delete, so we need to use malloc to avoid a
mismatch.

This fixes issue #579.
6 jobs for master
Status Job ID Name Coverage
  Test
canceled #3882
amd64
job_clang_amd64

canceled #3886
amd64
job_clang_amd64_nosh

canceled #3884
amd64
job_clang_py2_amd64

canceled #3881
amd64
job_gcc_amd64

canceled #3885
amd64
job_gcc_amd64_nosh

canceled #3883
amd64
job_gcc_py2_amd64