After the introduction of Lua 5.3 to OpenWrt, elektra picks it up instead
of 5.1. Since 5.1 is unsupported anyway, switch to 5.3.
Added liblua to PKG_BUILD_DEPENDS as the FindLua script is broken. Having
it here allows compilation to continue.
Signed-off-by: Rosen Penev <rosenp@gmail.com>