Use SHA256 constant when available

Should help with HHVM, re googleapis#153

Author: Ian Barber

src/Google/Signer/P12.php

@@ -82,7 +82,8 @@ public function sign($data)
           "PHP 5.3.0 or higher is required to use service accounts."
       );
     }
-    if (!openssl_sign($data, $signature, $this->privateKey, "sha256")) {
+    $hash = defined("OPENSSL_ALGO_SHA256") ? OPENSSL_ALGO_SHA256 : "sha256";
+    if (!openssl_sign($data, $signature, $this->privateKey, $hash)) {
       throw new Google_Auth_Exception("Unable to sign data");
     }
     return $signature;

src/Google/Verifier/Pem.php

@@ -64,7 +64,8 @@ public function __destruct()
    */
   public function verify($data, $signature)
   {
-    $status = openssl_verify($data, $signature, $this->publicKey, "sha256");
+    $hash = defined("OPENSSL_ALGO_SHA256") ? OPENSSL_ALGO_SHA256 : "sha256";
+    $status = openssl_verify($data, $signature, $this->publicKey, $hash);
     if ($status === -1) {
       throw new Google_Auth_Exception('Signature verification error: ' . openssl_error_string());
     }