Make RSpec examples pass

This pull request makes the current RSpec examples pass.  Problems fixed:

* Dflat::Version::Full#remove and Dflat::Version::Delta#remove treated Checkm::Manifest#remove as a chainable method, but that method returns an Array.

* The calls to Checkm::Manifest#add used `:base => foo_path` to specify base path, but Checkm::Manifest requires `:options => {:path => foo_path}` instead.

* One example had the typo "provider" for "producer".

* The gemspec was mildly invalid, and the Gemfile did not use HTTPS for the rubygems source.