Update CI to test against 5.3.0 beta

5.3.0-work2 has been obsoleted by the newer 5.3.0-beta

The TravisCI scripts currently break with luajit and lua-5.3.0 because
of luarocks issues.

Updating to luarocks-2.2.0 caused luarocks to search for "env lua"
instead of "env luajit" and there's a whole other mess of things
happening with lua-5.3.0-beta at the moment.

So, the code works, but the testing is broken for luajit and 5.3.0-beta.
Fix 64 bit integer processing on 32 bit platforms

This is the solution to mattsta/lua-cmsgpack#4 but rewritten
to use compile-time pointer size determination.