[dist] README.SETUP: memcached is mandatory now

memcached is not optional anymore it is mandatory for the webUI to work.
[api][backend] log buildhistory of patchinfos and check last version got build before release

fixes bnc#879793
[dist] update gem versions
[backend] Optimize sourcedeletion function

Optimization done in runtime at revfile rewrite process and do
not fail if a project was deleted during (long) run