No more rails 4 deprecation warnings

This PR results in there being only one remaining Rails4 deprecation warning (which pertains to [`secret_key_base`](http://edgeguides.rubyonrails.org/upgrading_ruby_on_rails.html) and should not be fixed until the point at which we migrate to Rails 4). 

The main types of deprecation warnings fixed here include:
- Deprecation of `Model.scoped` -- replace with `Model.all`
- Use `ActiveRecord::QueryMethods.references`
- rspec deprecates `expect { }.not_to raise_error(SomeSpecificError)` and requires a general `expect { }.not_to raise_error()` 

__Note:__
- There is some noise in the specs relating to `sh: convert: command not found`, `unknown encoding ASCII-8BIT` 
- There is a failing spec (for Rails4) `spec/components/post_revisor_spec.rb:209` - this failure should exist on the current master branch (it is not introduced by this PR)