lock Travis to Node.js v11

.travis.yml

@@ -1,14 +1,19 @@
 dist: xenial
 language: node_js
-node_js: node
+node_js:
+  - '11'
 
 cache:
   directories:
     - node_modules
     - public
 
-install:
-  - npm i
+# will run `npm install` automatically here
+
+before_install:
+  - sudo apt-get -qq update
+  - sudo apt-get install python3-pip
+  - sudo -H pip3 install --upgrade pip
 
 before_script:
   # https://docs.codeclimate.com/docs/travis-ci-test-coverage