Update ext.campaigner.php on line 706

In ExpressionEngine 2.8.1 clicking on the settings button of the Campaigner extension it shows this error:

Fatal error: Call to undefined method EE_Javascript::generate_json() in httpdocs/ee/system/expressionengine/third_party/campaigner/ext.campaigner.php on line 706

and it makes it unusable.

By replacing $this->EE->javascript->generate_json with json_encode it makes it usable again.