can set minimum app version. api filters out errors with app-version less than this amount. filtering only happens if both min_app_version and app-version are set.
changed names and logic of should_keep? to match kb article http://help.airbrake.io/kb/ios/app-versions
add tests for ErrorReport#should_keep?
Merge pull request #621 from CruGlobal/issue-621

Ignore errors reported from old versions of an app