patch 8.2.2733: detecting Lua version is not reliable

Problem:    Detecting Lua version is not reliable.
Solution:   Add "vim.lua_version". (Ozaki Kiichi, closes #8080)
diff --git a/ci/if_ver-1.vim b/ci/if_ver-1.vim
index d5b2bb6..adc40a7 100644
--- a/ci/if_ver-1.vim
+++ b/ci/if_ver-1.vim
@@ -6,7 +6,7 @@
   echo "*** Interface versions ***\n"
 
   echo 'Lua:'
-  PrintVer lua print(_VERSION)
+  PrintVer lua print(vim.lua_version, jit and "(LuaJIT)" or "")
 
   echo 'MzScheme:'
   PrintVer mzscheme (display (version))