Linguist doesn't support nested file extensions

This partially addresses an issue with the way the classifier handles (or rather doesn't) nested extensions.

[`test_presence`](https://github.com/github/linguist/blob/master/test/test_samples.rb#L58-L73) already checks that there are samples present for any non-unique file extension. Due to an inconsistency in the way we handle extensions in the classifier and the test suite this test doesn't fail for nested file extensions (such as `.pom.xml`).

This is short term fix for the current behaviour to reject nested extensions in `languages.yml` - another PR coming shortly to adapt the classifier to handle nested extensions properly.

/ cc @bkeepers @vmg