change the default branch of the repo

[stof]

Currently, the master branch of this repo contains the code for the Flysystem 1.x adapter. The 2.x branch contains the code for the Flysystem 2.x adapter which is a subtree spit of the main repo and has a different namespace than the 1.x code.
As Packagist displays the readme of the default branch, I think it would make sense to make it show the 2.x adapter.