Fix cachegrab_gui issue

[kikones34]

Installing the latest version of wxPython (4.0.4) yields the following error when executing cachegrab_gui:

  File "/usr/local/lib/python2.7/dist-packages/cachegrab-0.1.0-py2.7.egg/cachegrab/gui/dataset_explorer.py", line 56, in __init__
    grid = wx.GridSizer(3, 2)
TypeError: GridSizer(): arguments did not match any overloaded call:
  overload 1: not enough arguments
  overload 2: argument 2 has unexpected type 'int'
  overload 3: not enough arguments
  overload 4: not enough arguments

Updating the call to include a third gap argument fixes this problem. An alternative solution would be to specify in requirements.txt the specific version of wxPython which must be used.