Include details on how to run against `master`.

A very common issue is when a user attempts to run against the `master`
branch. Simply pointing to `master` doesn't work. This adds the steps
necessary to use `master`.

[ci skip]
Also, `rspec` isn't needed...the gem itself is empty.  It's just a meta gem that allows you to run `gem install rspec` (or put `gem 'rspec'` in your Gemfile) and get all the gems.  If you're individually specifying all the gems (as is the case here), then you don't need the rspec metagem.