chore: add active NodeJS versions to travis

- 8.x and 6.x

.travis.yml

@@ -1,6 +1,6 @@
 language: node_js
 before_install:
-- npm install -g grunt-cli
+- npm install -g grunt-cli@0.4
 script:
 - grunt --stack travis
 email:
@@ -18,6 +18,8 @@ matrix:
     - PUBLISH=true
     - secure: pLTzghtVll9yGKJI0AaB0uI8GypfWxLTaIB0ZL8//yN3nAEIKMhf/RRilYTsn/rKj2NUa7vt2edYILi3lttOUlCBOwTc9amiRms1W8Lwr/3IdWPeBLvLuH1zNJRm2lBAwU4LBSqaOwhGaxOQr6KHTnWudhNhgOucxpZfvfI/dFw=
     - secure: yERYCf7AwL11D9uMtacly/THGV8BlzsMmrt+iQVvGA3GaY6QMmfYqf6P6cCH98sH5etd1Y+1e6YrPeMjqI6lyRllT7FptoyOdHulazQe86VQN4sc0EpqMlH088kB7gGjTut9Z+X9ViooT5XEh9WA5jXEI9pXhQJNoIHkWPuwGuY=
+  - node_js: '8'
+  - node_js: '6'
 cache:
   directories:
   - node_modules