Use stable release of mPDF 7.0 (#256)

Author: KacerCZ

composer.json

@@ -46,7 +46,7 @@
         "squizlabs/php_codesniffer": "^2.7",
         "phpunit/phpunit": "^5.7",
         "dompdf/dompdf": "^0.8.0",
-        "mpdf/mpdf": "7.0.0-RC1@dev",
+        "mpdf/mpdf": "^7.0.0",
         "friendsofphp/php-cs-fixer": "^2.7"
     },
     "suggest": {