allow successful TravisCI runs in forks (go-sql-driver#639)

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

@@ -90,4 +90,5 @@ script:
   - go test -v -covermode=count -coverprofile=coverage.out
   - go vet ./...
   - test -z "$(gofmt -d -s . | tee /dev/stderr)"
+after_script:
   - $HOME/gopath/bin/goveralls -coverprofile=coverage.out -service=travis-ci