fix script/local to work with .travis.yml changes

& raise an error when a version can’t be found
Throw error when gem adapters loaded w/out correct/any version

This won’t help #3128 since initializers are loaded before routes, but
it would’ve made the solution to #3131 clear.
update AA::Dependencies to use method_missing

so it can handle arbitrary gem names without having to be set up first
Dependencies -> Dependency
update filter hooks to work when Devise is missing

https://github.com/gregbell/active_admin/pull/3041#issuecomment-42960939
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