docs: correct build status badge

Currently Travis CI is not used, so I replaced it with the Github Actions badge

README.md

@@ -11,8 +11,8 @@
     <img src="https://packagephobia.now.sh/badge?p=svelte" alt="install size">
   </a>
 
-  <a href="https://travis-ci.org/sveltejs/svelte">
+  <a href="https://github.com/sveltejs/svelte/actions">
-    <img src="https://api.travis-ci.org/sveltejs/svelte.svg?branch=master"
+    <img src="https://github.com/sveltejs/svelte/workflows/CI/badge.svg"
          alt="build status">
   </a>