<bugfix>Fix bug with obtaining port spec signature</bugfix> <ticket>#613</ticket>
<bugfix>Fix bug with obtaining port spec signature</bugfix>
<ticket>#613</ticket>

core/vistrail/port_spec.py:
  - PortSpec._get_signature: add return statement

hjanime/VisTrails,Nikea/VisTrails,Nikea/VisTrails,minesense/VisTrails,minesense/VisTrails,celiafish/VisTrails,Nikea/VisTrails,VisTrails/VisTrails,VisTrails/VisTrails,VisTrails/VisTrails,hjanime/VisTrails,celiafish/VisTrails,minesense/VisTrails,hjanime/VisTrails,minesense/VisTrails,celiafish/VisTrails,hjanime/VisTrails,minesense/VisTrails,VisTrails/VisTrails,VisTrails/VisTrails,celiafish/VisTrails,Nikea/VisTrails,hjanime/VisTrails