Upgrade rspec from 2.14 to 3.1.0

Move to the latest version of rspec to keep project up to date with
test framework. Purposely left `its` and `should` in the project where
convenient.
Remove 'stub' from test helpers

To prevent deprecation warnings on clients that make use of the
RabbitFeed test helpers, refactor helpers to use 'allow' rather than
'stub'. Also refactored test helpers to follow expected directory
structure and simplify spec configuration.
Update remaining gems
Lock rspec at >= 2.14.0 and < 3.2.0

RabbitFeed has been tested and works with these versions of rspec
including the current version: 3.1.0
Update the performance benchmark based on latest gem update