pixel.py: Updated ilastik command-line for latest release.

The command-line API for ilastik has changed.  I've modified the appropriate function in `gala/pixel.py`. I tested these changes via the following command:
```
./bin/gala-pixel --config-file=example/pix-config.json /tmp
```
 which seems to work just fine.  Let me know if there's some better test to try.

NOTE: I also updated the ilastik.cmake build script in BuildEM today.  This gala change is coupled to the latest rev of BuildEM, so update your gala checkout and BuildEM build in tandem.