Fix (auto-package-update-now)

As stated in a comment in #6, (package-menu--find-upgrades) only works inside
the Package buffer opened by (package-list-packages).  The explicit call of
(package-refresh-contents) was also removed, because (package-list-packages)
does this automatically.
Merge pull request #11 from rtrn/add-smoke-test

Fix (auto-package-update-now)