Use 'npm ci' instead of 'npm install'.

This does not tamper with the package.lock file.

Author: alanz

.travis.yml

@@ -22,7 +22,7 @@ before_install:
       /usr/bin/Xvfb :99 -screen 0 1024x768x24 > /dev/null 2>&1 &
     fi
 install:
-- npm install
+- npm ci
 - npm run vscode:prepublish
 
 script: