Build: Don't install grunt-cli manually in Travis

It's not needed as npm scripts have node_modules/.bin prepended to PATH.
It's also better to have all versions as locked up as possible to prevent build
instability.

Author: mgol

.travis.yml

@@ -1,5 +1,3 @@
 language: node_js
 node_js:
   - "8"
-before_script:
-  - npm install -g grunt-cli