add go cover to install list

.travis.yml

@@ -7,6 +7,7 @@ install:
   - go get code.google.com/p/go.tools/cmd/goimports
   - go get github.com/golang/lint/golint
   - go get code.google.com/p/go.tools/cmd/vet
+  - go get code.google.com/p/go.tools/cmd/cover
 language: go
 go:
   - 1.3