allow successful TravisCI runs in forks

Most forks won't be in goveralls and so this command in travis.yml was,
previously, failing and causing the build to fail.

Now, it doesn't!

Author: jmhodges

.travis.yml

@@ -86,4 +86,4 @@ script:
   - go test -v -covermode=count -coverprofile=coverage.out
   - go vet ./...
   - test -z "$(gofmt -d -s . | tee /dev/stderr)"
-  - $HOME/gopath/bin/goveralls -coverprofile=coverage.out -service=travis-ci
+  - $HOME/gopath/bin/goveralls -coverprofile=coverage.out -service=travis-ci || exit 0