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)

docs/DEVELOPER-ADVANCED.md

@@ -20,7 +20,7 @@ to rails, you are likely much better off with our **[Discourse Vagrant Developer
 2. `rake db:migrate`
 3. `rake db:test:prepare`
 4. `rake db:seed_fu`
-5. Try running the specs: `bundle exec rspec`
+5. Try running the specs: `bundle exec rake spec`
 6. `bundle exec rails server`
 
 You should now be able to connect to rails on http://localhost:3000 - try it out! The seed data includes a pinned topic that explains how to get an admin account, so start there! Happy hacking!

lib/tasks/rspec.rake

@@ -0,0 +1,3 @@
+require 'rspec/core/rake_task'
+
+RSpec::Core::RakeTask.new(:spec)