Use --prefer-source because of GitHub quota

[1] showed Composer\Downloader\TransportException during Travis initialization
which according to [2] is because of a GitHub quota

[1] https://github.com/SemanticMediaWiki/SemanticMediaWiki/pull/47

[2] https://github.com/travis-ci/travis-core/issues/186
Merge pull request #50 from SemanticMediaWiki/travis

Use --prefer-source because of GitHub quota