Remove unsupported gems from Gemfile

This removes Gems for the unsupported JRuby and SQLite.

I think we should somehow specify the required ruby version, but I'm not sure on how to do this.
* Bundler (with `ruby` in the Gemfile) only allows to specify a x.x.x release (e.g. 2.1.1), but not a x.x release like 2.1. I don't think we want to do this.
* A `.ruby-version` file is used by rvm, rbenv and others to automatically select the right ruby version. For rvm, this automatically deselects a Gemfile one might use and I don't want to always create a `.ruby-gemset` file, so I don't like this solution either.

@ulferts @myabc Do you have any ideas on how to do this in a better way? Should we merge this anyway?