Relax version constraint on JSON to ~> 1.7.6

This fixes a number of tickets including:

- [CHEF-3824](http://tickets.opscode.com/browse/CHEF-3824)
- [CHEF-2960](http://tickets.opscode.com/browse/CHEF-2960)
- [CHEF-3712](http://tickets.opscode.com/browse/CHEF-3712)