Revert "Update Ruby specified in the Gemfile to 2.1.1."

This reverts commit 5d262d35a66a2153d0fe5d28fde7be070c14f779. For reasons to be determined, installing Ruby 2.1 packages is not putting `ruby2.1` binaries in the list of ruby alternatives, which makes it tough/impossible to actually use the new ruby. In the meantime, plain ol' 2.0 should be fine.