Fix broken BerksfileCheck spec

This was executing `git check-ignore`, which is only available as of git
1.8. Since most CentOS systems are still on 1.7, this results in a
broken check most of the time (and as a result, a broken spec).

Fix it by getting the list of ignored files via an alternate method.

Change-Id: I2e6a1877e944112419c5fea9d7f0acd15be60402
Reviewed-on: http://gerrit.causes.com/37938
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>