Generate API doc on tag only

Because it doesn't need to be run on every PR and documenting
only release is exactly what we want

.travis.yml

@@ -38,7 +38,7 @@ jobs:
         - php ocular.phar code-coverage:upload --format=php-clover tests/coverage-clover.xml
 
     - stage: API documentations
-      if: branch = master
+      if: tag is present
       php: 7.4
       before_script:
         - curl -LO https://github.com/phpDocumentor/phpDocumentor/releases/download/v3.0.0-rc/phpDocumentor.phar