cache the bundle on travis.
Remove empty importing_test.
Move repository_test to rspec.

The context 'saving a file' was already moved, but not deleted from
the _test.rb file.
Move autocomplete_test to rspec.
Remove empty basic_proof_test
Move category_test to rspec.
Remove empty comment_test
Move formality_level_test to rspec.
Move graph_data_fetcher_test to rspec.
Remove empty language_adjoint_test.
Remove empty language_mapping_test.
Move language_population_test to rspec.
Move license_model_test to rspec.
Move link_test to rspec.
Remove empty link_version_test
Remove empty logic_adjoint_test
Remove empty logic_mapping_test
Move logic_population_test to rspec.
Move logic_test to rspec.
Remove empty metadata_test.