Allow people to not install 37 different rubies

I appreciate that Ruby is fairly core to work at GDS for some people,
however they do take a rather long time to compile and honestly, not
everyone needs all these versions.

This commit:
- moves ruby::version installs to a module
- allows overriding the installed versions in hiera
- removes non-essential ruby versions from @samjsharpe

I'd like to thank @norm for inspiring me to do this.
We're running 1.6.5 in prod now I think. Worth bumping...?