Fix waiting for apps in click_next_page() method

http://qa-selenium.mv.mozilla.com:8080/job/marketplace.dev.developer_hub.saucelabs.jquery_fix/6/testReport/junit/tests.desktop.developer_hub.test_developer_hub/TestDeveloperHub/test_that_a_screenshot_can_be_added/

An update to https://github.com/mozilla/marketplace-tests/pull/414