Avoid be_falsey and be_truthy

Created by: marcandre

This PR first fixes Configuration#any_predicate? to return true/false instead of any_predicate.

No spec fail for this change, because all specs on those predicates use be_truthy, like this one which passed even though config.custom_option? used to be a simple alias for config.custom_option and return 'a value'.

Second commit removes all uses of be_truthy and be_falsey except the two for testing the matchers themselves. I believe most of these date from before they were renamed to avoid confusion

Most of these changes are trivial be_truthy => be(true). A non-trivial example is this spec that was not updated now that fail-fast defaults to 1 instead of true.