Run unit tests with ginkgo installed from godep

Otherwise there's a good chance that the ginkgo you have installed will
have incompatible types with the ones we vendor with Godep.
Make bin/test more robust against ginkgo upgrades breaking the build

We found that when we upgrade ginkgo in our godeps, our tests tend to go
red and stay red for a while (typically until someone will ssh into a
box and poke around until it's fixed). The solution tends to involve

* upgrading the ginkgo binary
* removing the .a files in the $GOPATH/pkg directory

The previous commit assures we run the ginkgo binary from godep. This
commit ensures we clean out the .a files from the previous test run.