Update to 2.1.x ruby

I tried to run admin-ui with ruby 2.1 to make it compatible with the ruby packages from the latest bosh release (cloudfoundry-community/admin-ui-boshrelease#7).

All tests are passing, the only one problem was caused by old version of `eventmachine`. This is known issue (eventmachine/eventmachine#248) and the only solution I see is to go to a newer version. I fixed it by editing `Gemfile.lock`.