Ruby version installation errors and warnings

[schneems]

Currently when a customer tries to upgrade their stack and are using an old version of Ruby they get a very unhelpful error:

       Command: 'set -o pipefail; curl -L --fail --retry 5 --retry-delay 1 --connect-timeout 3 --max-time 30 https://s3-external-1.amazonaws.com/heroku-buildpack-ruby/heroku-18/ruby-1.9.3.tgz -s -o - | tar zxf - ' failed on attempt 1 of 3.
       Command: 'set -o pipefail; curl -L --fail --retry 5 --retry-delay 1 --connect-timeout 3 --max-time 30 https://s3-external-1.amazonaws.com/heroku-buildpack-ruby/heroku-18/ruby-1.9.3.tgz -s -o - | tar zxf - ' failed on attempt 2 of 3.
 !
 !     An error occurred while installing ruby-1.9.3
 !
 !     Heroku recommends you use the latest supported Ruby version listed here:
 !     https://devcenter.heroku.com/articles/ruby-support#supported-runtimes
 !
 !     For more information on syntax for declaring a Ruby version see:
 !     https://devcenter.heroku.com/articles/ruby-versions
 !
 !
 !     Debug InformationCommand: 'set -o pipefail; curl -L --fail --retry 5 --retry-delay 1 --connect-timeout 3 --max-time 30 https://s3-external-1.amazonaws.com/heroku-buildpack-ruby/heroku-18/ruby-1.9.3.tgz -s -o - | tar zxf - ' failed unexpectedly:
 !
 !     gzip: stdin: unexpected end of file
 !     tar: Child returned status 1
 !     tar: Error is not recoverable: exiting now
 !
 !     Push rejected, failed to compile Ruby app.
 !     Push failed

This is awful and gives no context. The goal of this PR is to do a few things:

• If a Ruby version does not exist on any stack, explicitly state the problem
• If a Ruby version exists, but not on your current stack, list the stacks it is present on
• If a Ruby version exists, but not for the next stack (i.e. heroku-16 if you're deploying to cedar-14) then the customer should get a warning

[edmorley]

Looks great, thank you!

[schneems]

Thanks for the review