Update travis script to use npm ci as well

.travis.yml

@@ -4,7 +4,10 @@ group: travis_latest
 
 language: python
 
-cache: pip
+cache:
+  - pip
+  directories:
+    - "$HOME/.npm"
 
 python:
 - 3.7
@@ -59,7 +62,7 @@ script:
 
 # Check JS code. The npm install must happen AFTER the flake8 because the node_modules folder will cause a lot of errors.
 - cd monkey_island/cc/ui
-- npm i
+- npm ci  # See https://docs.npmjs.com/cli/ci.html
 - eslint ./src --quiet  # Test for errors
 - JS_WARNINGS_AMOUNT_UPPER_LIMIT=490
 - eslint ./src --max-warnings $JS_WARNINGS_AMOUNT_UPPER_LIMIT  # Test for max warnings