Release aruba/rspec as a standalone gem without cucumber

[caalberts]

Summary

Release aruba-rspec gem which contains only relevant code for aruba/rspec.

Expected Behavior

When I need to use aruba with RSpec, I should be able to install the gem without cucumber.

Given the following Gemfile:

gem 'aruba-rspec'

When bundle install

Then Gemfile.lock should contain:

    aruba (1.1.0)
      childprocess (>= 2.0, < 5.0)
      contracts (~> 0.16.0)
      rspec-expectations (~> 3.4)
      thor (~> 1.0)

Current Behavior

Given the following Gemfile:

gem 'aruba'

When bundle install

Then Gemfile.lock should contain:

    aruba (1.1.0)
      childprocess (>= 2.0, < 5.0)
      contracts (~> 0.16.0)
      cucumber (>= 2.4, < 7.0) # cucumber is also installed
      rspec-expectations (~> 3.4)
      thor (~> 1.0)

Possible Solution

Context & Motivation

When aruba/rspec is used, there doesn't seem to be a need for the cucumber gem to be installed as well. Releasing aruba-rspec on its own would reduce the bundle size.

[mvz]

Please see #402 for previous discussion about splitting aruba.

The name aruba-rspec is taken, unfortunately, but apart from that, I'm all for this.

Pull requests welcome, but they'll have to include a migration path.