[bazbremner] Use newer ruby-build; ruby 2.1.5.
[bazbremner] Use newer ruby-build; ruby 2.1.5.

I only need 1.8.7-* and 2.1.5, uninstall the rest. Stock ruby-build on
goes to 2.1.2.
alphagov/gds-boxen,alphagov/gds-boxen,alphagov/gds-boxen