Issue title: Grouped prices show 0.00
 Issue body: http://forum.jigoshop.com/discussions/problems/581-price-listed-as-000-on-some-grouped-products-when-all-simple-products-are-priced

 Comments: 
 Comment 0: Already fixed in RHR

 Comment 1: Ah no way. I was just spending time trying to fix this damn thing lol. Glad you got it!

 Comment 2: Yep, will try & apply a patch over as soon as I can. Won't be tonight though I hope thats okay with you guys?

 Comment 3: I don't see any complaints. Just let us know when it's over :)

{'source': 'GithubIssues', 'repo_name_with_owner': 'jigoshop/jigoshop', 'issue_id': 2613175, 'repo_stars': 249.0}