lz4 2.1.3

Build more wheels

on Python PyPI

This release brings:

• A simplification of the tox.ini file (thanks to @jdufresne)
• More robust checking for pkgconfig availability (#158, thanks to @raymondEhlers)
• Integration of cibuildwheel into travis builds so as to build and publish binary wheels for Linux and OSX (fixes #99 , #144 , #153)
• Only require pytest-runner if pytest/test is being called (fixes #161)
• Blacklists version 3.3.0 of pytest which has a bug that can cause the tests to fail.