Fix deprecations for RSpec 3

As long as rspec-rails is not locked at any version, 3.1 is used. Therefore deprecation warnings (and errors) are showed in the console when running tests. Specifically, `be_true` is not longer a valid method - it either should be `be_truthy` or `be true` for a strict match, the same applies to `be_false`. This PR removes most of them (there are a couple more but they are related to CanCan itself).

Also please note that two tests are failing (`/spec/features/login_spec.rb`, lines 9 and 16) with the message "Could not find an executable ["phantomjs"] on your path." - it was already failing before I made any changes.