VersionChecker#missing_dependencies and #dependencies_satisfied?

Implements a few methods to perform extended upstream / downstream
cross-analysis and detection, comparing the specific requirements
in a gemfile to those in the configured build targets
Merge pull request #121 from movitto/missing_deps

VersionChecker#missing_dependencies and #dependencies_satisfied?
Fix: Duplicate version checker targets so as not to modify orig
Fix: check for block existance before yielding

Also return [:unknown] from unknown_versions helper
Fix: handle edge case where parsed yum version is nil
Fix: propery modify result array in dependency_states method
Add specs for version checker and versioned dependency modules