Improvement: Display a plugin's post install message if available

This change checks if the `Gem::Specification` was given a post install message (`spec.post_install_message = "Thanks for installing!"`). If it's found it is printed to the screen using `env[:ui].info`. This helps plugins that need just a bit of extra set up explaint to the user. See my latest pull request on Bindler [fgrehm/bindler#14](https://github.com/fgrehm/bindler/pull/14).