Merge branch 'version-plist'

Version checks for some OS X applications that don't support a
--version option.  Andrew Walker confirmed that this found his Safari
version on OS X 10.6.8, but the code is otherwise untested [1].

[1]: https://github.com/swcarpentry/workshop-template/pull/108#issuecomment-71517251

* version-plist:
  Fix decorator in VersionPlistCommandDependency
  swc-installation-test-2.py: Convert PathCommandDependency to VersionPlistCommandDependency