Add a Py3.7 travis build setup that uses gcc-7.
Add a Py3.7 travis build setup that uses gcc-7.

cython/cython,scoder/cython,da-woods/cython,da-woods/cython,cython/cython,cython/cython,scoder/cython,cython/cython,scoder/cython,da-woods/cython,scoder/cython,da-woods/cython