Added utility method to open a gem's homepage

Hi,

I often find myself either googling for a gem or doing 'gem search -d' whenever I want to see if there's a new version available, issues, recent activity etc. 
To avoid these cumbersome steps, I've added a utility method to open a bundled gem's homepage:

    # bundle goto rack

The $BROWSER env variable must be set for this action. Typically 'open' for OSX and 'xdg-open' for Linux.