Fix some rspec 3 deprecations

Just a small PR to fix some deprecation warnings in the spec suite.

* Updates the rspec matcher interface to use failure_message_when_negated
instead of negative_failure_message.
* Fix Rspec stub deprecation warning message 
* fixed up some trailing whitespace and indentation issues in files I touched. Most prominently `spec/matchers/break_down_to.rb`

This is similar to #693 although also fixes the issue in mail's test suite and also fixes the stub issue too.

I've run the suite against Ruby 2.1.2 and jruby 1.7.12 (1.9.3) since I noticed travis failed some specs on jruby recently.