add CentOS7 compatibility

CentOS / RHEL 7 use GCC 4.85, which does not support C++14. However, it doesn't look like your code uses anything from C++14, only C++11, which GCC 4.85 does support.

I tested changing setup.py so that extra_compile_args are -std=c++11 instead of -std=c++14 and it appears to work.
