Travis: simpify PHPStan build

As there is already a PHP 7.4 build in place running exactly the same build as the PHP 7.4 PHPStan build, with the exception of the actual PHPStan run, we can skip the "normal" build checks for the PHPStan build and just and only run PHPStan.

.travis.yml

@@ -30,6 +30,10 @@ jobs:
         apt:
           packages:
             - libonig-dev
+      script:
+      - composer require --dev phpstan/phpstan
+      - php vendor/bin/phpstan analyse --configuration=phpstan.neon
+
     # Nightly is PHP 8.0 since Feb 2019.
     - php: nightly
       addons:
@@ -83,5 +87,3 @@ script:
   - if [[ $XMLLINT == "1" ]]; then diff -B ./src/Standards/PSR12/ruleset.xml <(xmllint --format "./src/Standards/PSR12/ruleset.xml"); fi
   - if [[ $XMLLINT == "1" ]]; then diff -B ./src/Standards/Squiz/ruleset.xml <(xmllint --format "./src/Standards/Squiz/ruleset.xml"); fi
   - if [[ $XMLLINT == "1" ]]; then diff -B ./src/Standards/Zend/ruleset.xml <(xmllint --format "./src/Standards/Zend/ruleset.xml"); fi
-  # Run PHPStan
-  - if [[ $PHPSTAN == "1" ]]; then composer require --dev phpstan/phpstan && php vendor/bin/phpstan analyse --configuration=phpstan.neon; fi