Show specific upgrade instructions based on environment

Because ruby-build is both a Homebrew formula and an rbenv plugin, some people end up with both installed. In these cases, executing `rbenv install` vs. `ruby-build` might not use the same install. Users end up being unsure where `rbenv install` is coming from and upgrading it from the wrong location.

- https://github.com/sstephenson/ruby-build/issues/588#issuecomment-48026552
- https://github.com/sstephenson/ruby-build/issues/569#issuecomment-42778989
- https://github.com/sstephenson/ruby-build/pull/480
- https://github.com/sstephenson/ruby-build/issues/366#issue-15051163
- https://github.com/sstephenson/ruby-build/issues/362#issuecomment-19513846
- https://github.com/sstephenson/ruby-build/issues/254#issuecomment-16306018

This detect whether ruby-build is contained in Homebrew or git and shows upgrade instructions accordingly. Example:

    See all available versions with `rbenv install --list'.

    If the version you need is missing, try upgrading ruby-build:

      brew update && brew upgrade ruby-build

How does everyone feel about this?