add rubygems-2.1.11 md5

Fix error:

There is no checksum for 'http://production.cf.rubygems.org/rubygems/rubygems-2.1.11.tgz' or 'rubygems-2.1.11.tgz', it's not possible to validate it.
This could be because your RVM install's list of versions is out of date. You may want to
update your list of rubies by running 'rvm get stable' and try again.