travis: test against Go1.7

Fix new go vet complaints

.travis.yml

@@ -3,6 +3,7 @@ language: go
 go:
   - 1.5.4
   - 1.6.3
+  - 1.7
 
 go_import_path: google.golang.org/grpc
 
@@ -17,5 +18,5 @@ script:
   - '! gofmt -s -d -l . 2>&1 | read'
   - '! goimports -l . | read'
   - '! golint ./... | grep -vE "(_string|\.pb)\.go:"'
-  - '! go tool vet -all . 2>&1 | grep -vE "constant [0-9]+ not a string in call to Errorf"'
+  - '! go tool vet -all . 2>&1 | grep -vE "constant [0-9]+ not a string in call to Errorf" | grep -vF .pb.go:' # https://github.com/golang/protobuf/issues/214
   - make test testrace