Update the default branch to main

Update the reference to CI

Author: thagikura

README.md

@@ -1,5 +1,5 @@
 # FlexboxLayout
-[ ![Circle CI](https://circleci.com/gh/google/flexbox-layout.svg?style=shield&circle-token=2a42716dfffab73d73c5ce7ed7b3ee620cfa137b) ](https://circleci.com/gh/google/flexbox-layout/tree/master)
+[ ![Circle CI](https://circleci.com/gh/google/flexbox-layout.svg?style=shield&circle-token=2a42716dfffab73d73c5ce7ed7b3ee620cfa137b) ](https://circleci.com/gh/google/flexbox-layout/tree/main)
 [ ![Download](https://api.bintray.com/packages/google/flexbox-layout/flexbox/images/download.svg) ](https://bintray.com/google/flexbox-layout/flexbox/_latestVersion)
 
 FlexboxLayout is a library project which brings the similar capabilities of