Keep development files out of the Composer packages

To reduce download size, we should have a `.gitattributes` file that makes sure that development-only files (tests, CI stuff and non-production configuration) will not end up in the Composer packages and the ZIPs generated by GitHub.

Like this: https://github.com/MyIntervals/emogrifier/commit/80266bb671b4aab6c8d496230546b902ec0d3e1a#diff-fc723d30b02a4cca7a534518111c1a66