Add bundle:update command

![2014-04-03 12 37 35](https://cloud.githubusercontent.com/assets/9955/2599437/5be02f90-bae1-11e3-9acc-8635a485e114.png)

HI,
Thank you for your nice package :heart: 
I using everyday, but I wanna use `bundle update` too.

How about this patch?