Adds .gitattributes that excludes paths when installed via --prefer-dist

Adds `.gitattributes` in response to recommendations below.

See https://groups.google.com/forum/#!topic/thephpleague/jyPyZJPwImc
See https://www.reddit.com/r/PHP/comments/2jzp6k/i_dont_need_your_tests_in_my_production/