chore: remove travis ci

And add a buildkite build status badge

Author: mokou

.travis.yml

@@ -1,6 +1,6 @@
 ![crab pet](https://i.imgur.com/LbZJgmm.gif)
 
-# rustlings 🦀❤️
+# rustlings 🦀❤️ [![Build status](https://badge.buildkite.com/7af93d81dc522c67a1ec8e33ff5705861b1cb36360b774807f.svg)](https://buildkite.com/mokou/rustlings)
 
 Greetings and welcome to `rustlings`. This project contains small exercises to get you used to reading and writing Rust code. This includes reading and responding to compiler messages!