Change default branch to v1.6.3

[per1234]

At this point I think the majority of Arduino users are using IDE version 1.6.x and enough time has passed for any serious bugs in the branch to have been found.

[Andy2No]

I was wondering how best to put this, and I now see the issue was already created, 7 years ago.

After following a link to this repo, then trying to install as described, nothing new appeared in the Tools->Board menu, in Arduino 1.8.19

As above, the solution is to install it from branch v1.6.3, not the default v1.06, which is no longer compatible.

If anyone is still maintaining this repo, the default branch does indeed need to be changed to v1.6.3, otherwise installation does not work.