back out r11486

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11487 e93f8b46-1217-0410-a6f0-8f06a7374b81
Ruby2.0: add "require 'yaml'" to Gemfile

On ruby 2.0.0p0 (2013-02-24 revision 39474) [x86_64-linux]:
<pre>
$ ruby script/about
There was an error in your Gemfile, and Bundler cannot continue.
</pre>

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11488 e93f8b46-1217-0410-a6f0-8f06a7374b81
Ruby2.0: add platforms to Gemfile mysql2

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11489 e93f8b46-1217-0410-a6f0-8f06a7374b81