Pass `--linter=rubocop --test=rspec` to `bundle gem`

[sambostock]

The generator expects to patch RSpec files, so specifying anything other than rspec as the test framework would break generation. Therefore, it makes sense to specify it ourselves, and save the user the time of having to select it.

It also seems very unlikely that someone would want to generate a RuboCop extension, but use a linter other than RuboCop, so let's save the user the time.

[sambostock]

This turns out to be similar to #10, but takes a different approach, by forcing RSpec, rather than checking for it.