Specify compatible gem versions

I did some testing on different versions of the gems required by RabbitFeed to determine on which versions RabbitFeed is compatible. I've updated the gemspec to reflect this.

Upgraded all gems used by RabbitFeed in the process, including rspec, which required changes to the tests to support the latest rspec version. On this, I found that RabbitFeed test helpers were causing deprecation warnings in the example apps, so I changed the helpers so that they no longer throw these warnings.

This change will require that clients update their `spec_helper.rb`.

Resolves https://github.com/simplybusiness/rabbit_feed/issues/4 and https://github.com/simplybusiness/rabbit_feed/issues/6

@sparrovv @calo81 Could you please sign off?