Update default branch to 'main'

Author: tavianator

README.md

@@ -3,8 +3,8 @@
 
 [![crates.io](https://img.shields.io/crates/v/acap.svg)](https://crates.io/crates/acap)
 [![Documentation](https://docs.rs/acap/badge.svg)](https://docs.rs/acap)
-[![License](https://img.shields.io/badge/license-MIT-blue.svg)](https://github.com/tavianator/knn/blob/master/LICENSE)
-[![Build Status](https://travis-ci.com/tavianator/acap.svg?branch=master)](https://travis-ci.com/tavianator/acap)
+[![License](https://img.shields.io/badge/license-MIT-blue.svg)](https://github.com/tavianator/knn/blob/main/LICENSE)
+[![Build Status](https://travis-ci.com/tavianator/acap.svg?branch=main)](https://travis-ci.com/tavianator/acap)
 
 As Close As Possible — [nearest neighbor search] in Rust.