Precompile CKEditor plugins in use.

As stated in #1196, compiling everything related to CKEditor made
compilations slower. However, not compiling any plugins meant Travis had
to compile them while running a test. It often resulted in a test
failing because the time Travis took to compile the plugins the
application uses exceeded Capybara's wait time.

config/initializers/ckeditor.rb

@@ -1,4 +1,4 @@
 Ckeditor.setup do |config|
   config.assets_languages = Rails.application.config.i18n.available_locales.map{|l| l.to_s.downcase}
-  config.assets_plugins = []
+  config.assets_plugins = %w[copyformatting tableselection scayt wsc]
 end