Move behat.yml to behat.yml.dist

behat config can require changes to match the current environment (most notably the hostname/url),
as well as any other custom configurations.

For this reason, ideally it should be kept outside of git repo, and instead use a default/dist file
(much in the same way as ezpublish.yml)

This changes the file name, updates travis script accordingly, and updates .gitignore.