Display versions of uninstalled distributions

It's helpful to see the versions uninstalled for auditing purposes and
if the user realizes they broke something and they want to restore
things back to exactly how they were.

Before:

    $ pip uninstall pyrepl pytest
    Uninstalling pyrepl:
      Successfully uninstalled pyrepl
    Uninstalling pytest:
      Successfully uninstalled pytest

After:

    $ pip uninstall pyrepl pytest
    Uninstalling pyrepl 0.8.4:
      Successfully uninstalled pyrepl 0.8.4
    Uninstalling pytest 2.6.4:
      Successfully uninstalled pytest 2.6.4
This just uses the string representation of the dist, which I'm not keen on. What if that changes?

I'd rather that the format of the output were explicit here, so we kept control of it. Also, I think we need to ensure consistency with #2221 so I'd rather we agreed on the format once and used it consistently in both PRs (actually, it may be better to merge the two PRs into one, so discussions on the message format get covered in one place - but that's less important than getting consistency, however we do that).