Specify bundler version

This allows specifying an exact version of Bundler to install with a particular Ruby version:

```puppet
rbenv::compile { "2.0.0-p0":
  user    => "lmars",
  bundler => "1.3.5"
}

rbenv::compile { "1.9.3-p194":
  user    => "lmars",
  bundler => "1.0.22"
}
```