Use RubyGems version 2.1.11

Version 2.2.0 breaks things. What kind of things? These:

    vagrant@precise64:/vagrant$ bundle
    Unfortunately, a fatal error has occurred. Please see the Bundler
    troubleshooting documentation at http://bit.ly/bundler-issues. Thanks!
    /usr/local/lib/ruby/2.1.0/fileutils.rb:250:in `mkdir': Permission denied @ dir_s_mkdir - /usr/local/lib/ruby/gems/2.1.0/extensions/x86_64-linux (Errno::EACCES)

And:

    vagrant@precise64:/vagrant$ gem
    Error loading RubyGems plugin "/usr/local/lib/ruby/gems/2.1.0/gems/yard-0.8.7.3/lib/rubygems_plugin.rb": Permission denied @ dir_s_mkdir - /usr/local/lib/ruby/gems/2.1.0/extensions/x86_64-linux (Errno::EACCES)

And:

    vagrant@precise64:/vagrant$ sudo gem update --system 2.1.11
    Error loading RubyGems plugin "/usr/local/lib/ruby/gems/2.1.0/gems/yard-0.8.7.3/lib/rubygems_plugin.rb": ERROR: Failed to build gem native extension.
    # ...
    /bin/bash: line 10: cd: /home/vagrant/.bundler/tmp/11865/gems/ffi-1.9.3/ext/ffi_c/libffi: No such file or directory
    make[1]: *** [/home/vagrant/.bundler/tmp/11865/gems/ffi-1.9.3/ext/ffi_c/libffi/Makefile.in] Error 1
    make[1]: Leaving directory `/usr/local/lib/ruby/gems/2.1.0/gems/ffi-1.9.3/ext/ffi_c/libffi-x86_64-linux'
    make: *** ["/usr/local/lib/ruby/gems/2.1.0/gems/ffi-1.9.3/ext/ffi_c/libffi-x86_64-linux"/.libs/libffi_convenience.a] Error 2
    make failed, exit code 2
    Gem files will remain installed in /usr/local/lib/ruby/gems/2.1.0/gems/ffi-1.9.3 for inspection.
    Results logged to /usr/local/lib/ruby/gems/2.1.0/extensions/x86_64-linux/2.1.0-static/ffi-1.9.3/gem_make.out
     (Gem::Ext::BuildError)