Make travis file simpler (#386)

* Make travis file simpler

the are two reasons
1. Since of May 2019 by default start Ubuntu Xenial 16.04
https://blog.travis-ci.com/2019-04-15-xenial-default-build-environment
2. and also since Ubuntu 16.04 you can start xvfb as services
https://docs.travis-ci.com/user/gui-and-headless-browsers/#using-xvfb-to-run-tests-that-require-a-gui

 - move xvfb to services
 - Remove update npm (it was need for old versions of node)

* travis executes export DISPLAY=:99.0 when service xvfb start and don't need repeat again

.travis.yml

@@ -3,18 +3,11 @@ node_js:
   - 8
   - 10
 
-# Make sure we have new NPM.
-before_install:
-  - export DISPLAY=:99.0
-  - sh -e /etc/init.d/xvfb start
-  - sleep 3 # Give xvfb time to to start
-  - npm install -g npm
-  - npm config set loglevel warn
+services:
+  - xvfb
 
 before_script:
   - npm install -g grunt-cli
 
 script:
-  - grunt
-
-dist: trusty # Impacts this script, eg xvfb
+  - grunt