Revert "Require rspec-rails in Gemfile to fix 'spec' task"

This reverts commit c6c223d1c9f47d756a08e8064b30e987751f72cd.
Add explicit 'spec' rake tasks

This doesn't require auto-loading of rspec-rails and puts the 'spec'
task in the rake task list (rake -T)
Merge pull request #858 from chrishunt/chrishunt/create-explicit-spec-task

Create 'spec' rake task