Use prefer-dist unstead of prefer-source in Travis

[pascalchevrel]

Github has removed the API rating limit when installing dependencies from packages instead of clones:
composer/composer#4884 (comment)

That means we should now use prefer-dist instead of prefer-source which is faster since it downloads the snapshot of code from the tag and not the whole repo with history.

[pascalchevrel]

This PR almost halved the build time on Travis, let's hope it's not just luck :)