Show links to Ruby installation guides for OSX, etc.

[e2]

Scenario:

1. User has e.g. OSX 10.10 with outdated Ruby 2.0.0
2. RubyDep shows warning about outdated Ruby, suggests versions to migrate to and ...
3. ... shows links to pages/instructions where user can e.g. use homebrew or rvm/rbenv to install an updated Ruby