Use new "cover" path

The official "cover" package has moved, and Go 1.4 will start enforcing
canonical imports.

.travis.yml

@@ -6,7 +6,7 @@ go:
 install:
     - go list -f '{{range .Imports}}{{.}} {{end}}' ./... | xargs go get -v
     - go list -f '{{range .TestImports}}{{.}} {{end}}' ./... | xargs go get -v
-    - go get code.google.com/p/go.tools/cmd/cover
+    - go get golang.org/x/tools/cmd/cover
     - go build -v ./...
 script:
     - go test -v -cover ./...