Use failure_message/failure_message_when_negated rspec matchers

[mzahir]

Gets rid of the deprecation warnings

`failure_message_for_should_not` is deprecated. Use `failure_message_when_negated` instead. 
Called from ~/.rbenv/versions/2.1.6/lib/ruby/gems/2.1.0/gems/websocket-rails-0.7.0/lib/spec_helpers/matchers/route_matchers.rb:37:in `block in <top (required)>'.

`failure_message_for_should` is deprecated. Use `failure_message` instead.
 Called from ~/.rbenv/versions/2.1.6/lib/ruby/gems/2.1.0/gems/websocket-rails-0.7.0/lib/spec_helpers/matchers/route_matchers.rb:33:in `block in <top (required)>'.

[recurser]

👍 I was about to submit a PR for the same thing

[recurser]

+1