Add BerksfileCheck pre-commit hook

Add pre-commit hook which checks if your `Berksfile.lock` is out of sync
with your `Berksfile` via the `berks list` command.

This helps avoid situations where you forget to commit updates to your
`Berksfile.lock`.

Change-Id: I026992d6af1522a14806ee4f88bb426776d18f20
Reviewed-on: http://gerrit.causes.com/37805
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Shane da Silva <shane@causes.com>