Print a warning if not using LuaJIT

[thibaultcha]

We should print a warning when an instance is spawned and not using LuaJIT. Basically running on bare Lua will be very slow. Throwing some benchmarks: http://luajit.org/performance_x86.html

if jit then
  -- LuaJIT
else
  -- Not LuaJIT (slower)
end

@SGrondin I hope your benchmarks are using LuaJIT :p

[subnetmarco]

The warning can't be launched in the CLI because the CLI always uses the system Lua.
I have added a lua_version property at 127.0.0.1:8001/ that shows what version of Lua is being used by OpenResty.

The newer version of OpenResty are compiled by default always with LuaJIT. Because of this, I think we can close the issue.

[thibaultcha]

I never said the warning should be printed from the CLI. (The CLI can also use LuaJIT btw, it's my setting).