Compile with Ruby 2.3, not 2.1 (to fix saves)

I found out what the actual issue was with saves — it wasn’t that
xScripts needed to be recompiled (although it helped load previous
saves from the Windows build on Steam), it was that Ruby 2.1 doesn’t
seem to work with the saves, @mathewv might have determined this when
selecting 2.2 as the minimum.  I have Ruby 2.3 on my system, installed
through Homebrew, so that is what I selected for my particular build.

mkxp.pro

@@ -244,7 +244,7 @@ BINDING_NULL {
 
 BINDING_MRI {
 	isEmpty(MRIVERSION) {
-		MRIVERSION = 2.1
+		MRIVERSION = 2.3
 	}
 
 	PKGCONFIG += ruby-$$MRIVERSION