prefer-source in composer in .travis.yml so that it uses git

Travis is thrashing the github api, which causes transport errors when composer
tries to get packages from it. By specifying --prefer-source, composer should
use git instead of the github api, which should stop our builds from failing.

See https://github.com/travis-ci/travis-core/issues/186
Merge pull request #361 from voidus/travis

Fix travis build