Change instructions to recommend installing rc5

The documentation says to install rc2.  However, rc2 doesn't include the generator for integration tests (SpecsGenerator).

Users will need to install a newer version of the gem to run:

    rails generate clearance:specs

as is recommended in the [Optional Integration tests](https://github.com/thoughtbot/clearance#optional-integration-tests) section.

Alternatively, the documentation could recommend using:

    gem 'clearance', :git => 'https://github.com/thoughtbot/clearance.git'