16572 - deb pkg installs to a ruby-version independent dir

As newer debian/ubuntu releases default to ruby 1.9, mcollective-common
should install files to a version-independent directory.

/usr/lib/ruby/vendor_ruby/ has been in ruby's default path on debian at
least since lenny (2009).
Merge pull request #114 from richardc/bug/master/16572

16572 - deb pkg installs to a ruby-version independent dir