Update CI to test against 5.3.0 beta

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

This testing infrastructure is taken from:
https://github.com/moteus/lua-vararg/tree/246dc3126dd66935a020a0a26c165ceb035268e5/.travis
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.
Replace checkint with checkinteger

one is an alias of the other, but one was removed in 5.3+, so it's
better to use the common function across all versions.