Now that we've configured pruning for dep, this removes all unused
packages, all non-go files, and all tests from the vendor directory.
NOTE: due to a change in dep, it preserves anything that looks like a
license file. We'll be pulling in a few files we weren't previously
using - mostly license files. It's easier to just go with what dep does
than to try to exclude them after the fact.
Signed-off-by: Andy Goldstein <andy.goldstein@gmail.com>