Improved exception for GuardClauseAssertion,

when verification fails for a method that returns Task (or is marked async).
Added GuardClauseAssertion tests for Task[<T>],

ensuring that correctly implemented guard clauses are verified correctly.
Hyperlink to #268 for task/async GuardClauseAssertion
Incremented patch version,

in order to indicate that providing a better error message when verifying
Guard Clauses for async (deferred) methods is an improvement, but not a
new feature per se. This resolves #268
(https://github.com/AutoFixture/AutoFixture/issues/268).