Bump df-core

Author: yaroslavmo

composer.json

@@ -29,7 +29,7 @@
   "minimum-stability": "dev",
   "prefer-stable": true,
   "require":     {
-    "dreamfactory/df-core":   "~0.19.1",
+    "dreamfactory/df-core":   "~0.20.0",
     "dreamfactory/df-system": "~0.4.0"
   },
   "autoload":    {