Update as upstream changed its branch structure for release version

[haxpor]

According to #15 (comment), and #16 , we found out that branch structure has been changed upstream. More detail at #16 (comment).

So in short, it's better to

• allow user to select either beta or normal release to build exposing to aseprite.sh's argument e.g. aseprite.sh --beta to build a beta version, otherwise build the latest normal release
• adjust branch URL accordingly as used by git commands in aseprite.sh in various places in aseprite.sh