travis updates

[jprjr]

Split out from #97 , updates for Travis

• Updates Lua 5.3
• Updates Luarocks
• Adds Lua 5.4
• Adds 32-bit builds of Lua 5.3 and Lua 5.4
• Uses latest lua-cjson module from git instead of luarocks
• Conditionally install luabitop (Lua 5.1 and Lua 5.2 only)
• Explicitly checks out the LuaJIT branch to be tested (upstream now defaults to 2.1, previously if LuaJIT 2.0 was requested it was actually testing against 2.1)

[leafo]

Thanks for the contributions. I'm thinking about this and going forward I would prefer to migrate to the lua github actions for testing the lua variants, and migrate away from travis entirely:

https://github.com/leafo/gh-actions-lua https://github.com/leafo/gh-actions-luarocks

postgres could be loaded in with another action for integration testing

In the meantime though I think if it works then we can merge it in, but I don't want to invest too much energy on the travis scripts

[jprjr]

That makes sense, yeah. It should work, I think the current build failed just because #98 wasn't merged in yet with the fix for checking os.execute

[leafo]

So it seems like travis is on its way out even faster, so I migrated over to github actions. All the travis code has now been removed from the repo. Most of the issues listed here are resolved with the exception of support for testing on 32bit Lua.

That's going to have to be a patch to the Lua action here to allow build flags to be passed: https://github.com/leafo/gh-actions-lua

Thanks for the patch, sorry I wasn't able to accept it as is, but at least we've now got much better Lua coverage