update tests for more strict erchef requirements

* erchef is now more strict when it comes to cookbook constraint validations.
  environment cookbook constraint tests updated to reflect that
  nil version, empty array, and single-element array should be rejected
* some minor refactoring for readability and to reduce number of pended
  tests.
* erchef now accepts version in format "1.0.0" with no qualifier
Merge pull request #41 from opscode/ma/oc-6988

update tests for disallowed nil version and version w/out qualifier