[5.5] Add an everyFifteenMinutes function to the ManagesFrequencies t…

…rait t… (#21092)

* Add an everyFifteenMinutes function to the ManagesFrequencies trait to allow scheduling every 15 minutes.

* styleci fix

* styleci fix again.

* styleci

src/Illuminate/Console/Scheduling/ManagesFrequencies.php

@@ -347,6 +347,16 @@ public function everyTenMinutes()
         return $this->spliceIntoPosition(1, '*/10');
     }
 
+    /**
+     * Schedule the event to run every fifteen minutes.
+     *
+     * @return $this
+     */
+    public function everyFifteenMinutes()
+    {
+        return $this->spliceIntoPosition(1, '*/15');
+    }
+
     /**
      * Schedule the event to run every thirty minutes.
      *