Don't load rake task unless on a supported Ruby

The `flatui-pro:convert` Rake task is the only one in this gem (or `bootstrap-sass`) that gets loaded directly into the Rails app.

It depends on several scripts in `bootstrap-sass` that otherwise don't get loaded. [One of these](https://github.com/twbs/bootstrap-sass/blob/master/tasks/converter/less_conversion.rb#L174) uses keyword arguments, a language feature only supported on MRI 2.x.

This wraps the loading of the Rake task in a conditional, so people still on MRI 1.9.3 (or other rubies, like Rubinius) can still use the rest of the gem.