Change how 'debops.ruby' role manages Ruby version

From now on, Ruby will fall back on APT to select the version of Ruby
packages to install - if Ruby 2.1 is available, APT should automatically
install it; if not, APT will install Ruby packages already present in
the distribution.

Ruby backports can be enabled by setting ruby_version: 'backport'.
This functionality might be moved in the future to a separate playbook.
Install 'rubygems-integration' package separately

This package might not exist on older versions of Debian/Ubuntu
distributions.
Don't look for installed gems if gem list is empty

When there are no gems specified to install, role will not check the
list of installed gems. You can also disable or change default list of
gems via inventory.
Merge pull request #1 from debops/ruby_version_overhaul

Ruby version overhaul