use git clean to remove cruft (.pyc etc) after checkout

@feanil or @e0d 

to avoid errors caused by leftover .pycs, such as this recent one:

```
Traceback (most recent call last):
  File "/opt/edx/bin/django-admin.py", line 5, in <module>
    management.execute_from_command_line()
  File "/opt/edx/local/lib/python2.7/site-packages/django/core/management/__init__.py", line 443, in execute_from_command_line
    utility.execute()
  File "/opt/edx/local/lib/python2.7/site-packages/django/core/management/__init__.py", line 382, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/opt/edx/local/lib/python2.7/site-packages/django/core/management/base.py", line 196, in run_from_argv
    self.execute(*args, **options.__dict__)
  File "/opt/edx/local/lib/python2.7/site-packages/django/core/management/base.py", line 231, in execute
    self.validate()
  File "/opt/edx/local/lib/python2.7/site-packages/django/core/management/base.py", line 266, in validate
    num_errors = get_validation_errors(s, app)
  File "/opt/edx/local/lib/python2.7/site-packages/django/core/management/validation.py", line 30, in get_validation_errors
    for (app_name, error) in get_app_errors().items():
  File "/opt/edx/local/lib/python2.7/site-packages/django/db/models/loading.py", line 158, in get_app_errors
    self._populate()
  File "/opt/edx/local/lib/python2.7/site-packages/django/db/models/loading.py", line 64, in _populate
    self.load_app(app_name, True)
  File "/opt/edx/local/lib/python2.7/site-packages/django/db/models/loading.py", line 88, in load_app
    models = import_module('.models', app_name)
  File "/opt/edx/local/lib/python2.7/site-packages/django/utils/importlib.py", line 35, in import_module
    __import__(name)
  File "/opt/wwc/edx-platform/lms/djangoapps/shoppingcart/models.py", line 183, in <module>
    class PaidCourseRegistration(OrderItem):
  File "/opt/wwc/edx-platform/lms/djangoapps/shoppingcart/models.py", line 188, in PaidCourseRegistration
    mode = models.SlugField(default=CourseMode.DEFAULT_MODE_SLUG)
AttributeError: type object 'CourseMode' has no attribute 'DEFAULT_MODE_SLUG'
```