Agent should update to the latest Plugin version in some cases

Currently the agent will download a plugin for something like `blah-blah-plugin` from the master branch the first time it needs it, but it won't update it ever again.

This means that go get your changes as you are iterating, you end up updating your pipeline to reference specific commits like `blah-blah-plugin#{commit-hash}`.

The meta-effect of this is that it's very hard to get your plugin users hotfixes on master. They need to delete the plugin or change their pipeline.

It would be great to fetch the latest version on each job if the plugin isn't a ref or a symbolic ref /cc @sj26.