Incremental check for coding standards in pull requests

Fixes #2917.

.travis.yml

@@ -257,7 +257,15 @@ jobs:
         - wget https://scrutinizer-ci.com/ocular.phar
         - php ocular.phar code-coverage:upload --format=php-clover clover.xml
 
+    - stage: Pull request coding standard
+      if: type = pull_request
+      php: 7.2
+      script:
+        - wget https://github.com/diff-sniffer/git/releases/download/0.1.0/git-phpcs.phar
+        - php git-phpcs.phar origin/$TRAVIS_BRANCH...$TRAVIS_PULL_REQUEST_SHA
+
     - stage: Coding standard
+      if: NOT type = pull_request
       php: nightly
       script:
         - ./vendor/bin/phpcs