Update compatibility check to latest LÖVR API

lovr-keyboard.lua

@@ -1,5 +1,5 @@
 local ffi = assert(type(jit) == 'table' and               -- Only run if we have LuaJIT
-  lovr.getOS() ~= 'Android' and lovr.getOS() ~= 'Web' and -- and also GLFW
+  lovr.system.getOS() ~= 'Android' and lovr.system.getOS() ~= 'Web' and -- and also GLFW
   require 'ffi', "lovr-keyboard cannot run on this platform")
 local C = ffi.os == 'Windows' and ffi.load('glfw3') or ffi.C