As @Tim already mentioned - this problem was discussed before already. This post gives the proper solution - just tested it in 14.04 and it is still valid & working
The following is a 100% quote from here.
Open a terminal by pressing Ctrl+Alt+T and type the following commands:
sudo dpkg-reconfigure wireshark-common
press the right arrow and enter for yes
sudo chmod +x /usr/bin/dumpcap
you should now be able to run it without root and you will be able to capture.