Replaced usage of deprecated Common::json_encode to json_encode

core/Common.php

core/Plugin/MetadataLoader.php

core/Tracker.php

core/Translate.php

core/Updates/1.8.3-b1.php

plugins/CustomVariables/Archiver.php

plugins/Dashboard/Controller.php

plugins/Dashboard/Dashboard.php

plugins/Goals/Controller.php

plugins/MobileMessaging/API.php

plugins/Overlay/Controller.php

plugins/SEO/MajesticClient.php

plugins/ScheduledReports/API.php

plugins/ScheduledReports/Controller.php

plugins/UserCountry/Controller.php

plugins/UserCountryMap/Controller.php

plugins/Widgetize/Controller.php

tests/PHPUnit/Fixtures/UITestFixture.php

tests/PHPUnit/TestingEnvironment.php