Unxfail test_marketplace_search_and_install_app.py
update locator for wallpaper_preview
Merge pull request #1018 from bebef1987/fix_walppaper

update locator for wallpaper_preview
Merge pull request #1013 from teodosia/remove-xfail-install-app

Unxfail test_marketplace_search_and_install_app.py
Bumping marionette_client dependency to 0.5.31, to pick up greatness; r=jgriffin