Update CircleCI env to Node 8

Author: fathyb

.circleci/config.yml

@@ -2,7 +2,7 @@ version: 2
 jobs:
   test:
     docker:
-      - image: circleci/node:4-browsers
+      - image: circleci/node:8-browsers
     steps:
       - checkout
       - run: npm config set "//registry.npmjs.org/:_authToken" $NPM_AUTH
@@ -18,7 +18,7 @@ jobs:
           path: junit-reports
   publish:
     docker:
-      - image: circleci/node:4-browsers
+      - image: circleci/node:8-browsers
     steps:
       - checkout
       - attach_workspace: { at: . }