avoid TypeError when reading the active io

I ran into the following error while trying to slice with the current code on a raspberry pi running Debian:

```
Traceback (most recent call last):
  File "/usr/lib/python2.7/runpy.py", line 162, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/home/brad/projects/Cura/Cura/cura.py", line 89, in <module>
    main()
  File "/home/brad/projects/Cura/Cura/cura.py", line 77, in main
    data = gcode.read()
  File "Cura/util/bigDataStorage.py", line 25, in read
    ret = self._active.read(size)
TypeError: an integer is required
```

This PR is my proposed solution to fix the problem.