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.
Change gds_ruby parameter name

deprecated isn't the right word, so I've settled on uninstall_versions
[alexmuller] Add Hiera for removing Rubies
Merge pull request #111 from alphagov/no-rubby

Allow people to not install 37 different rubies