Fix Fatal TypeErrors in toString overrides

Author: fishfacemcgee

src/Constraint/CrawlerNot.php

@@ -31,7 +31,7 @@ protected function fail($nodes, $selector, ComparisonFailure $comparisonFailure
         );
     }
 
-    public function toString()
+    public function toString() : string
     {
         if ($this->string) {
             return 'that contains text "' . $this->string . '"';

src/Constraint/JsonContains.php

@@ -56,7 +56,7 @@ protected function matches($other) : bool
      *
      * @return string
      */
-    public function toString()
+    public function toString() : string
     {
         //unused
         return '';

src/Constraint/JsonType.php

@@ -50,7 +50,7 @@ protected function matches($jsonArray) : bool
      *
      * @return string
      */
-    public function toString()
+    public function toString() : string
     {
         //unused
         return '';

src/Constraint/WebDriverNot.php

@@ -32,7 +32,7 @@ protected function fail($nodes, $selector, ComparisonFailure $comparisonFailure
         );
     }
 
-    public function toString()
+    public function toString() : string
     {
         if ($this->string) {
             return 'that contains text "' . $this->string . '"';