Correctly mark allowed failures for non-Ruby projects

We use Job#matrix_config? to test if an allow_failures entry matches a
job, which gets the keys to compare from Build.matrix_keys_for. Since
the allow_failures entry doesn't include a language key,
Build.matrix_lang_keys only returns the keys included that are "Ruby"
keys, since Ruby is the default language. This in turn makes the config
that matrix_config? tests against empty, causing no jobs to be marked as
allowed failures.
Merge pull request #307 from travis-ci/hh-fix-allow-failures-with-custom-language

Correctly mark allowed failures for non-Ruby projects