Declare Maybe destructor implicitly

GCC 4.8.1 rejects implicit exception-specification in an explicitly
defaulted destructor declaration if the destructor allows some
exception. As a result, some instantiations of the Maybe class template
don't compile. As a workaround, the destructor of Maybe is totally
implicitly declared for now.
Merge pull request #21 from magicant/fix/maybedestructor

Declare Maybe destructor implicitly