fix implementation of get_display()

Author: oesteban

nipype/utils/config.py

@@ -309,7 +309,7 @@ def _mock():
             ndisp = sysdisplay.split(':')[-1].split('.')[0]
             Xvfb = namedtuple('Xvfb', ['new_display', 'stop'])
             self._display = Xvfb(int(ndisp), _mock)
-            return sysdisplay
+            return self.get_display()
         else:
             if 'darwin' in sys.platform:
                 raise RuntimeError(
@@ -336,8 +336,7 @@ def _mock():
             if not hasattr(self._display, 'new_display'):
                 setattr(self._display, 'new_display',
                         self._display.vdisplay_num)
-
-            return ':%d' % self._display.new_display
+            return self.get_display()
 
     def stop_display(self):
         """Closes the display if started"""