Make linting rules more consistent

 * Put back babel-eslint for class-properties
 * Allow arrow functions without params

This makes the style more consistent with react-sdk.

NB. The line lengths are still inconsistent but it's not clear which
way to go on that yet.

.eslintrc.js

@@ -1,6 +1,6 @@
 module.exports = {
+    parser: "babel-eslint", // now needed for class properties
     parserOptions: {
-        ecmaVersion: 2017,
         sourceType: "module",
         ecmaFeatures: {
         }
@@ -72,5 +72,6 @@ module.exports = {
             "named": "never",
             "asyncArrow": "always",
         }],
+        "arrow-parens": "off",
     }
 }

package.json

@@ -66,6 +66,7 @@
   },
   "devDependencies": {
     "babel-cli": "^6.18.0",
+    "babel-eslint": "^10.0.1",
     "babel-plugin-transform-async-to-bluebird": "^1.1.1",
     "babel-plugin-transform-runtime": "^6.23.0",
     "babel-preset-es2015": "^6.18.0",