add 'service' goveralls flag

Author: jrperritt

.travis.yml

@@ -15,4 +15,4 @@ before_install:
   - if ! go get github.com/golang/tools/cmd/cover; then go get golang.org/x/tools/cmd/cover; fi
 script:
   - script/cibuild
-  - $HOME/gopath/bin/goveralls
+  - $HOME/gopath/bin/goveralls -service=travis-ci