More helpful error messages for be_true and be_false

These are a couple of helpful error messages when you try to use `be_true` and `be_false`. This would have been helpful for me when I started a new project using 3.0 and was surprised to find that `be_true` raised ```NoMethodError``` with the message ```undefined method `true?' for true:TrueClass```, which lead me to believe something was broken in my environment. This will hopefully save some time for other people in the same situation.