Fixed claim check in JwtValidator

* Removed the addRequiredClaimWithValue() method, as checking
  the value should be up to the implementation.

README.md

@@ -78,10 +78,8 @@ Parsing is done by passing the raw token as the 2nd parameter
     use NoccyLabs\SimpleJwt\Validator\JwtValidator;
 
     $validator = new JwtValidator();
-    // Require that the claim exists
+    // Require that some claim exists
     $validator->addRequiredClaim("some/required/Claim");
-    // Require that the claim exists and has a value of true
-    $validator->addRequiredClaimWithValue("some/required/OtherClaim", true);
 
     try {
         // Pass a JwtToken to validateToken()...