Explicitly enable object-rest-spread plugin

We need this because esprima, the JavaScript parser used by
electron-link, only supports ES2017, even though our current
Electron version supports some ES2018 features. Without this
plugin enabled, `npm run test:snapshot` fails with an esprima
error.

Author: smashwilson

.babelrc.js

@@ -5,10 +5,13 @@ module.exports = {
     "./assert-messages-plugin.js",
     "@atom/babel-plugin-chai-assert-async",
     "@babel/plugin-proposal-class-properties",
+
+    // Needed for esprima
+    "@babel/plugin-proposal-object-rest-spread",
   ],
   presets: [
     ["@babel/preset-env", {
-      "targets": {"electron": process.versions.electron}
+      targets: {electron: process.versions.electron}
     }],
     "@babel/preset-react"
   ],

package-lock.json

@@ -43,6 +43,7 @@
     "@atom/babel7-transpiler": "1.0.0-1",
     "@babel/generator": "7.3.4",
     "@babel/plugin-proposal-class-properties": "7.3.4",
+    "@babel/plugin-proposal-object-rest-spread": "7.3.4",
     "@babel/preset-env": "7.3.4",
     "@babel/preset-react": "7.0.0",
     "babel-plugin-relay": "3.0.0",