Remove unnecessary dependencies and support for older versions

[ydah]

What? Why?

I noticed that adding gruf-rspec to a Gemfile adds gems other than the rspec-core and rspec-expectations. Dependencies on only rspec-core and rspec-expectations are fine because they use only the following APIs:

• RSpec.configure
  • https://rspec.info/documentation/3.8/rspec-core/RSpec/Core/Configuration
• RSpec::Matchers.define
  • https://rspec.info/documentation/3.8/rspec-expectations/RSpec/Matchers

We also care for an older version, which I think is care up to rspec 1.3.2. This project relies on RSpec 3.8+, which I think is unnecessary. https://www.rubydoc.info/gems/rspec/1.3.2/Spec/Runner