Work toward Rails 4.2 beta support

This resolves #1147. It is mostly a hack until Ammeter is updated.

The reason that `capture` was removed from Rails is that it is not thread safe :unamused:. I have no idea if this hack is thread safe or not. I do know it works for now and makes the build pass. Since this is local to our specs, and uses the same technique as the built-in matcher (https://github.com/rspec/rspec-expectations/blob/master/lib/rspec/matchers/built_in/output.rb) I think it's "good enough" :grimacing: 

/cc @myronmarston @samphippen @JonRowe 
Hasn't this been release now?