Allow natural assertions to handle to_bool

If a Then/And/Invariant clause returns an object that responds to
:to_bool, then the to_bool method is called to determine true/false
status.
Remove failing require in failing examples.
Bump to version 3.3.0.beta1
Limit length of inspect string in error output
Bump to version 3.3.0.beta2
Update README to reference +to_bool+.
Updated README history
Bump to version 3.3.0
Article about custom failure messages in RSpec/Given.