chore(ci): stop testing golang "tip" since "go vet|cover" broke

Go 1.2 and 1.3 require this command to install "go vet":
  `go get code.google.com/p/go.tools/cmd/vet`
Go "tip" in Travis-CI requires this command:
  `go get golang.org/x/tools/cmd/vet`

Neither is happy with the other's package path, so let's not worry about testing golang "tip" for now.