update to phpspreadsheet 1.6.0

Author: erelke

composer.json

@@ -19,7 +19,7 @@
         "php": ">=7.0",
         "symfony/framework-bundle": "~3.2|~4.0",
         "twig/twig": "~2.0",
-        "phpoffice/phpspreadsheet": "~1.5.2"
+        "phpoffice/phpspreadsheet": "~1.6.0"
     },
     "require-dev": {
         "symfony/symfony": "~3.4|~4.0",