`jenkins_plugin` fixes/improvements

* Properly allow installation of specific versions of a plugin. Previously
this only worked when a source URL was provided.
* Ensure a plugin’s dependencies are installed before proceeding with
it’s installation
* Handle plugin downgrades correctly (requires an uninstall of existing,
newer version).

Fixes #260, #269, #271, #276
Some `META-INF/MANIFEST.MF` keys are missing values
Make installing a plugin’s deps optional
Clean up YARD docs