Fix the cuke so it works properly.

- Only manipulate the $LOAD_PATH in the rspec process,
  not in the aruba process.
- No need for the @manip-env tag, which would show up
  on relish.
- Use aruba’s `set_env` API.
- Append `--disable-gems` to RUBYOPT rather than replacing it.
- Since `expect` is primarily an rspec-expectations API, demonstrate
  the lack of rspec-mocks with `double` instead.
- No need for `--format progress`.
- Change require_expect_syntax_in_aruba_specs.rb to use the newer
  `disable_monkey_patching!` option rather than configuring expectations
  and mocks. Configuring those libs directly would fail when they
  have been removed from the load path, and `disable_monkey_patching!`
  provides the overall effect we want: monkey patching is removed
  from all pieces of RSpec we use.