Change the way rubygems-integration backport works

Due to recent changes in 'ruby2.1' package in Debian concerning
rubygems-integration dependency (more information:
http://anonscm.debian.org/cgit/collab-maint/ruby.git/commit/?id=96ed23a3ef80613043350d8a61916966186371b9)
backported 'ruby2.1' package cannot be installed on Wheezy with
correctly backported 'rubygems-integration' package - version
1.8~bpo76+1 is not sufficient for requirement >= 1.8.

Because of that, this patch changes how 'rubygems-integration'
backporting is handled. 'rubygems-integration' package will be
recompiled without any changes to the version number, changelog, etc.
technically not being a backport - it will have version 1.8 to satisfy
requirements.

Due to these changes, an APT pin is configured by the 'debops.ruby' role
that should configure APT to slightly prefer version of
'rubygems-integration' package from Debian Jessie, which should
facilitate smooth upgrade in the future, although this solution is not
yet tested at this point.