go vet yourself (#397)

go vet caught some nifty bugs. so fixed those here, and also made it so that
we vet everything from now on since the robots seem to do a better job of
vetting than we have managed to.

also adds gofmt check to circle. could move this to the test.sh script (didn't
want a script calling a script, because $reasons) and it's nice and isolated
in its own little land as it is. side note, changed the script so it runs in
100ms instead of 3s, i think find is a lot faster than go list.

attempted some minor cleanup of various scripts

api/agent/drivers/docker/docker_test.go

@@ -112,6 +112,6 @@ func TestRegistry(t *testing.T) {
 	}
 
 	if size <= 0 {
-		t.Fatalf("expected positive size for image that exists, got size:", size)
+		t.Fatal("expected positive size for image that exists, got size:", size)
 	}
 }