add CentOS7 compatibility

[jdratlif]

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.

[alex-petrenko]

I had to bump from 11 to 14 during the development at some point, but it can be the latest version of the code does not use anything C++-14 specific now. Could also be compiler dependent.

I would really appreciate if you submit a PR, then it will go through automated tests and I'll be happy to merge it!