Adding psych on Gemfile explicitly

It seems like bundle has a bug[1] that, somehow, it unloads the psych
library unless is installed through bundle itself. It will be fixed on
bundle 1.2.

[1]: http://github.com/bundler/bundler/issues/2068

Change-Id: Ic2fa8a8f114c3183a656bfdb1bc2d6d6413dbb75

Gemfile

@@ -14,6 +14,10 @@ group :development, :test do
   gem 'puppet-lint-numericvariable',          :require => 'false'
   gem 'json',                                 :require => 'false'
   gem 'webmock',                              :require => 'false'
+  # adding 'psych' explicitly
+  # https://github.com/bundler/bundler/issues/2068
+  # TODO: drop it in a future release of 'bundle'.
+  gem 'psych',                                :require => 'false'
 end
 
 group :system_tests do