[cron] Bump distribution date

Marlin/src/inc/Version.h

    * here we define this default string as the date where the latest release
    * version was tagged.
    */
-  #define STRING_DISTRIBUTION_DATE "2019-08-19"
+  #define STRING_DISTRIBUTION_DATE "2019-08-21"
 
   /**
    * Required minimum Configuration.h and Configuration_adv.h file versions.