Switch primary branch from `master` to `main` (#809)

See: #805

README.md

@@ -1,7 +1,7 @@
 Emscripten SDK
 ==============
 
-[![CircleCI](https://circleci.com/gh/emscripten-core/emsdk/tree/master.svg?style=svg)](https://circleci.com/gh/emscripten-core/emsdk/tree/master)
+[![CircleCI](https://circleci.com/gh/emscripten-core/emsdk/tree/main.svg?style=svg)](https://circleci.com/gh/emscripten-core/emsdk/tree/main)
 
 The Emscripten toolchain is distributed as a standalone Emscripten SDK. The SDK
 provides all the required tools, such as Clang, Python and Node.js along with an