Allow users to skip asset dependency checks

Similar to #84 we want to give users the option of not running these tests. 

Related but not covered here, the case where an asset in a gem is missing a dependency: https://github.com/schneems/sprockets_better_errors/issues/5

Adding the ability to at least stop the check from occurring would give developers the ability to continue to develop even if a Gem does not update their sprockets dependencies.