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.

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
Merge pull request #633 from sstephenson/upgrade-instructions

Show specific upgrade instructions based on environment