Update Haxe 3.1.3 OS X Installer url.

Currently the [Haxe 3.1.3 Mac OS X installer url](http://haxe.org/file/haxe-3.1.3-osx-installer.dmg) url from the readme is a broken link, redirecting to a 404 not found error page.

[Link updated](http://haxe.org/download/file/3.1.3/haxe-3.1.3-osx-installer.pkg) to the current Haxe package.