MCOP-184 Don't build package for distributions with no ruby 1.9

EL5 and 6 variants only have ruby 1.8 available.  As this agent was
coded against 1.9 for Process#spawn (and may have some other 1.9
assumptions) do not build packages for those.
Merge pull request #2 from richardc/MCOP-184_ruby_19_platforms

MCOP-184 Don't build package for distributions with no ruby 1.9