Fix issue in server class causing runtime error

The `nginx::server` class has a runtime error because the params that reference
`nginx::params` can't be read without inheriting from the params class.

It's a very ugly pattern that we should refactor out as soon as possible - we don't
really need a params class since we only support Ubuntu.

I've also fixed up the specs so that `bundle exec rake` works, and removed some unneeded gems from the `Gemfile`.