get ruby-eventmachine debian pacakge from sid into wheezy-backports

then we don't need to compile the gem and don't need the libssl-dev package on the server

quoting micah from oct. 11th:

Unfortunately, the ruby-eventmachine package that is available in wheezy was not built with libssl-dev. That wasn't done until the newer version which is in sid, but has not transitioned to jessie for some time. If we could get it to transition, then we could backport it and use it, but until then, we will have to continue to use the gem.

(from redmine: created on 2013-10-11, closed on 2015-05-26, precedes #3960 (closed))