attributes_table_for: check if association exists

resolves #2917
exclude gems from SimpleCov stats on Travis

https://github.com/lemurheavy/coveralls-public/issues/296
update travis_cache to work around travis-ci/travis-ci#2321

The S3 bucket permissions now allow downloading files anonymously, but
still require permissions to upload.
travis_cache: don't needlessly download app SHA
simplify CanCan adapter method
oops, don't raise an exception!