chore: Update Travis CI config to be able to work with docs

.travis.yml

@@ -1,5 +1,11 @@
 sudo: false
+dist: "trusty"  # otherwise 'python3-pip' won't be available
 language: "node_js"
+addons:
+  apt:
+    packages:
+      - "python3"
+      - "python3-pip"
 node_js:
   - "4"
   - "6"
@@ -15,10 +21,11 @@ before_install:
   # so 'conventional-changelog-lint' could compare commits and lint them: marionebl/conventional-changelog-lint#7
   - "git remote set-branches origin master && git fetch && git checkout master && git checkout -"
   - "npm -g install npm@latest"
+  - "pip3 install -r docs/requirements.txt"
 install: "npm install --no-optional"
 script:
   - "npm run lint"
-  - "npm run docs:lint"
+  - "npm run docs:build"
   - "npm run test:coverage"
 after_success:
   - "npm run coveralls || true"