update setDI / getDI function header

Author: Martin Vasel

Library/Phalcon/Test/Traits/UnitTestCase.php

@@ -176,11 +176,9 @@ public function getConfig()
      * @param  DiInterface $di
      * @return $this
      */
-    public function setDI(DiInterface $di)
+    public function setDI(DiInterface $di) : void
     {
         $this->di = $di;
-
-        return $this;
     }
 
     /**
@@ -189,7 +187,7 @@ public function setDI(DiInterface $di)
      * @see    Injectable::getDI
      * @return DiInterface
      */
-    public function getDI()
+    public function getDI() : DiInterface
     {
         if (!$this->di instanceof DiInterface) {
             return Di::getDefault();