Hightlight "installing" logs

Highlight logs like this if gems are newly being installed.
![image](https://f.cloud.github.com/assets/18807/1582228/298eff04-51ea-11e3-8815-f8c9e7a34e79.png)

I know calling private method must be avoided but I have no idea how to detect each gem is already installed or not.
Do you have any ideas? :octocat: