if you want a quick solution before the update from backbpx come
uninstall w3af (apt-get remove w3af)
download the latest version from github
cd /w3af
sudo ./w3af_gui
the script allow you to install all dependencies
after all finish
restart w3af_gui
if you want to add w3af on applications menu
RIGHT click menu > edit applications
click auditing => exploitation => web application
click add from the top of window click add launcher
set command (sudo ./w3af_gui)

set working directory to the w3af directory (example /home/you/w3af)
save exit
now you can start w3af from the menu