Skip mcrypt-related testcases on PHP 7.1

ext/mcrypt is deprecated and the test cases in question trigger E_DEPRECATED messages as a result.
diff --git a/tests/codeigniter/libraries/Encrypt_test.php b/tests/codeigniter/libraries/Encrypt_test.php
index ced7633..adbca31 100644
--- a/tests/codeigniter/libraries/Encrypt_test.php
+++ b/tests/codeigniter/libraries/Encrypt_test.php
@@ -10,6 +10,10 @@
 		{
 			return;
 		}
+		elseif (version_compare(PHP_VERSION, '7.1.0-alpha', '>='))
+		{
+			return $this->markTestSkipped('ext/mcrypt is deprecated since PHP 7.1 and will generate notices here.');
+		}
 
 		$this->encrypt = new Mock_Libraries_Encrypt();
 		$this->ci_instance_var('encrypt', $this->encrypt);
@@ -72,4 +76,4 @@
 		$this->assertEquals('cfb', $this->encrypt->get_mode());
 	}
 
-}
\ No newline at end of file
+}