reimplement travis ci using docker
tests now cover python 2.7, 3.4, 3.5, 3.6, 3.7, and assorted versions of dmd, ldc and gdc
also, get gdc to build extensions again
@9il these changes remove osx from .travis.yml; I looked in the build history a little and didn't see any osx builds, do you know if osx builds ever happened for this project?
reimplement travis ci using docker tests now cover python 2.7, 3.4, 3.5, 3.6, 3.7, and assorted versions of dmd, ldc and gdc also, get gdc to build extensions again