Add PyPy3 support

pyenv now supports PyPy3 (https://github.com/yyuu/pyenv/commit/ade2bf2a0da0ea7774b4e7dda6572f84caf25916)
Remove PyPy 2.3 support
Merge pull request #334 from travis-ci/ha-feature-pypy3

Add PyPy3 support