Fix PDFgui for OSX in 64-bit mode

This changes PDFgui icon by removing the 74x74 pixel layer. This is done to avoid error on 64-bit OSX using wxPython 2.9. The error and solution are described [here](http://trac.wxwidgets.org/ticket/13030).

This was tested on OSX 10.8.5, 64-bit Enthought Canopy (1.2.0.1576), wxPython 2.9.2.4-1.

I performed some manual tests, and this appears to be the only thing that is needed to get PDFgui to run with this configuration.