Don't test the case when a gist contains multiple blobs

When GitHub's gist API payload gets deserialized in Ruby, the file list
gets converted to a hash. Since hashes are unsorted in Ruby 1.8, we
can't predict which file will get picked as the "first file". Therefore,
avoid making such a test.

This means that applying patches from gists that have multiple files in
them will have unpredictable results on Ruby 1.8.