only-bundler.sh: Add "latest" support.
only-bundler.sh: Add "latest" support.

The "latest" argument can be passed to ensure the latest RubyGems and
Bundler versions are installed. Using "latest" for just the Bundler
version is the same as providing no argument, but can be useful in
Travis matrixes for readability.

chrismo/bundler-fixture,chrismo/bundler-fixture