Fixes shell issue when loading starting data

This PR fixes two issues introduced in https://github.com/nephila/djangocms-installer/pull/60 (No idea why this has been merged even though the tests were failing?)

* `subprocess.check_call(["python", "starting_page.py"], shell=True)` would cause python to open a shell with every argument provided (`python` and `starting_page.py`). You either have to remove `shell=True` (which is preferred) or provide the command as one string (`subprocess.check_call(["python starting_page.py"], shell=True)`).
* `os.remove('starting_page.pyc')`: `starting_page.pyc` is not always present -> raises OSException