Memoizer: save current revision number in memo file

The memo file is then invalidated if the saved revision number does not
match the current revision number.  This required bumping the internal
memo file version number from 1 to 2.
Add method to disable revision checking

Calling 'setVersionChecking(true)' will disable revision checking, but
keep major/minor version checking.  This may be useful for cases where
doing strict revision checking would cause an inordinate amount of
overhead.
Merge pull request #646 from melissalinkert/invalidate-cache

Save current revision number in memo file