Unpack manually and use release_handler:set_unpacked/2

cf) [erlang-bugs] release_handler error in upgrading slim release
    http://erlang.org/pipermail/erlang-bugs/2013-November/003900.html
Update README to include workaround
Merge pull request #1 from shino/manual-set-unpacked

Use `release_handler:set_unpacked/1` manually