Implements streaming CSVs.

Allows unlimited downloads of CSV data using ActiveRecord batch APIs and ActionController response streaming.

Works on Rails 3.2+
Actually pass the options to generate_line.
Make the meat and potatoes of streaming protected.
Add no-cache header to streaming CSV responses.
Disable downstream response buffering of stream
Add header_action method for panels
Use Capybara to better test panel rendering
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!
simplify options handling code for `actions`
style update
make `default_actions` private

note: this is a backwards-incompatible change!
changed spec syntax from should to expect
Merge pull request #3136 from irmiller22/rspec-syntax-change

RSpec Syntax Change
use expect() RSpec syntax in Cucumber steps
simplify ActiveAdmin::BatchActions::BatchActionPopover#action