chore(deps): bump github.com/darkrockmountain/gomail from 0.6.0-beta to 0.6.0 in /.examples/serverless/google_cloud_functions

chore(deps): bump github.com/darkrockmountain/gomail from 0.6.0-beta to 0.6.0 in /.examples/serverless/google_cloud_functions #269