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

The `rspec-rails` gem was set to `require = false` a few days ago in [`b6bf95e7`](https://github.com/discourse/discourse/commit/b6bf95e74183fbfb5fee73441247d80f52e2081a), which causes the spec rake task to silently fail. Notice that the [latest Travis build](https://travis-ci.org/discourse/discourse/jobs/7261893) is no longer running the specs.

This PR fixes the spec rake task.