Update eslint and .travis.yml (#259)

* simplify .travis.yml

* Fixes error and warning from eslint

15:32  error  Unnecessary escape character: \?  no-useless-escape

.travis.yml

@@ -1,7 +1,3 @@
 language: node_js
-node_js:
-    - "8"
+node_js: stable
 sudo: false
-
-script:
-    - "npm test"

devtools-panels/devtools/panel/devtools-panel.js

@@ -54,7 +54,7 @@ document.getElementById("button_background").addEventListener("click", () => {
 });
 
 const inspectString = "inspect(document.querySelector('h1'))";
-document.getElementById("button_h1").addEventListener("click", function(){
+document.getElementById("button_h1").addEventListener("click", () => {
     browser.devtools.inspectedWindow.eval(inspectString)
     .then(handleResult);  
 }); 

google-userinfo/background/authorize.js

@@ -12,7 +12,7 @@ const AUTH_URL =
 const VALIDATION_BASE_URL="https://www.googleapis.com/oauth2/v3/tokeninfo";
 
 function extractAccessToken(redirectUri) {
-  let m = redirectUri.match(/[#\?](.*)/);
+  let m = redirectUri.match(/[#?](.*)/);
   if (!m || m.length < 1)
     return null;
   let params = new URLSearchParams(m[1].split("#")[0]);

package.json

@@ -4,7 +4,7 @@
   "version": "1.0.0",
   "description": "Example Firefox add-ons created using the WebExtensions API",
   "devDependencies": {
-    "eslint": "^3.19.0"
+    "eslint": "^4.4.1"
   },
   "repository": {
     "type": "git",