(doc) Add documentation on rspec and mocha versions

Community members frequently experience issues running the specs because
of mismatched versions of RSpec and Mocha. This adds documentation to
the README_DEVELOPER.markdown and Gemfile to indicate the version
requirements and the reasons why those requirements are in place.
(maint) Move testing deps into Gemfile group
Merge pull request #1714 from adrienthebo/doc-mocha_version_requirements

(doc) Add documentation on rspec and mocha versions