.travis.yml: Fix Lua download link
.travis.yml: Fix Lua download link

The unofficial GitHub mirror became a bit more official and
removed/changed releases. Switch to the official Lua site instead.

Signed-off-by: Uli Schlachter <346c0ad49ac8427b61b3b55abbe37dc0ebd524f2@znc.in>

pavouk/lgi,psychon/lgi