Add buble to rollup

package.json

@@ -19,6 +19,7 @@
     "node-static": "0.6.0",
     "phantomjs-prebuilt": "^2.1.12",
     "rollup": "^0.34.10",
+    "rollup-plugin-buble": "^0.14.0",
     "rollup-watch": "^2.5.0"
   },
   "bugs": "http://github.com/codemirror/CodeMirror/issues",

rollup.config.js

+import buble from 'rollup-plugin-buble';
+
 export default {
   banner: `// CodeMirror, copyright (c) by Marijn Haverbeke and others
 // Distributed under an MIT license: http://codemirror.net/LICENSE
@@ -11,5 +13,6 @@ export default {
   entry: "src/codemirror.js",
   format: "umd",
   dest: "lib/codemirror.js",
-  moduleName: "CodeMirror"
+  moduleName: "CodeMirror",
+  plugins: [ buble() ]
 };