Added a `bundle licenses` cli task

This commit adds a `bundle licenses` task, which lists the license for each gem in the bundle or "Unknown" if one isn't found.

After having felt the pain of walking through a Ruby application's dependencies to determine license information on several occasions, it would be a dream to me if more gem authors used the `license` directive in their gemspecs, which would allow a simple task like this to be a more authoritative guide to determine dependency licenses.

I've added a new test for this change, and would love any and all feedback to this approach.

Thanks!