Also test build docs with Travis CI.

.travis.yml

@@ -31,6 +31,18 @@ matrix:
     - php: 5.6
       env: PREFER_LOWEST=1
 
+    - language: python
+      python: 2.7
+      env: DOCS=1
+      install:
+        - pip install -r docs/requirements.txt
+      addons:
+        apt:
+          packages:
+            - texlive-latex-recommended
+            - texlive-latex-extra
+            - texlive-fonts-recommended
+
 before_script:
   - if [[ $TRAVIS_PHP_VERSION != 7.1 ]]; then phpenv config-rm xdebug.ini; fi
 
@@ -50,6 +62,8 @@ script:
   - if [[ $PHPCS = 1 ]]; then vendor/bin/phpcs -p --extensions=php --standard=vendor/cakephp/cakephp-codesniffer/CakePHP ./src ./tests; fi
   - if [[ $PHPSTAN = 1 ]]; then vendor/bin/phpstan analyse -c phpstan.neon -l 4 src; fi
 
+  - if [[ $DOCS = 1 ]]; then cd docs && make html; fi
+
 after_success:
   - if [[ $DEFAULT = 1 && $TRAVIS_PHP_VERSION = 7.1 ]]; then bash <(curl -s https://codecov.io/bash); fi