Fix cache invalidation to behave properly.

Use `delete-file` rather than simply setting cache to `nil`.  nil is
still a valid cache value which can cause reinitialization to fail.

Fix bug with nil funcall.
Merge branch 'master' into invalidate-cache

Conflicts:
	sx-cache.el - RESOLVED
Merge pull request #80 from jleechpe/invalidate-cache

Fix cache invalidation to behave properly.