Run Godeps as a test

run godeps -t ./... as a test and ensure that dependencies.tsv agrees with what godeps outputs.

This will fail if a dev hasn't run godeps -u, it will also fail if someone adds a dependency on a new external repo and doesn't specify it in dependencies.tsv. 

The nice thing is, this is one of the first tests that'll run if you're doing go test ./... from the root directory, so it should alert you right away if something is wrong.

I also simplified the other test a little...tests always run in the current directory, so you don't have to bend over backwards to read a file in the same directory, just read it.

This has only one added precondition that didn't exist before - godeps must be in your path.  Everyone contributing must have godeps in order to create a valid development environment, but in theory someone may have it in some directory that is not in their path.  I think the added requirement is perfectly acceptable in order to add this level of insurance to our build.  

The other option is we could call it as $GOPATH/bin/godeps which would require you to have used go install, but doesn't force it to be in the path.  Not sure which would be deemed less offensive.
exec.LookPath, skip test if it doesn't exist