Remove Travis, Appveyor badges; add Buildkite badge
We migrated away from Travis and Appveyor onto Buildkite a while ago,
but forgot to update our build status badges.
<img src="https://github.com/habitat-sh/habitat/blob/master/www/source/images/habitat-logo.png" width="200">
-[![Build Status](https://api.travis-ci.org/habitat-sh/habitat.svg?branch=master)](https://travis-ci.org/habitat-sh/habitat)
-[![Build status](https://ci.appveyor.com/api/projects/status/ejn8d6bkhiml16al/branch/master?svg=true)](https://ci.appveyor.com/project/chef/habitat/branch/master)
+[![Build Status](https://badge.buildkite.com/f527cd3d7851756ed1a5f6ec463dd41e7145f7941fd932672a.svg)](https://buildkite.com/chef/habitat-sh-habitat-master-verify?branch=master)
[![Slack](http://slack.habitat.sh/badge.svg)](http://slack.habitat.sh/)
[![Discourse status](https://img.shields.io/discourse/https/meta.discourse.org/status.svg?style=flat)](https://forums.habitat.sh)
[![Open Source Helpers](https://www.codetriage.com/habitat-sh/habitat/badges/users.svg)](https://www.codetriage.com/habitat-sh/habitat)
