Bring 3.5 compatibility to windows

Currently, `PYTHON_VERSION=2.7` for `appveyor.yml`. During #531, @bsipocz discovered that halotools is not 3.5 compatible in windows environments. There is a single test failure, the `test_reader_configurations` function in `halotools/sim_manager/tests/test_rockstar_hlist_reader.py` raises a mysterious IndexError when run on AppVeyor in python 3.5. This test passes for all versions of python executed in a Unix environment, and this test also passes on AppVeyor windows environment for python 2.7.

When this test failure is resolved, and 3.5 compatibility is brought to windows, then `PYTHON_VERSION` should be set to `3.5` in `appveyor.yml`.