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 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.
Merge pull request #24 from mboersma/dont-test-golang-tip

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