Stop throwing errors when grading policy is missing min_count key

[pdpinch]

From https://odl.zendesk.com/agent/tickets/2401

8.05 has a degenerate but otherwise working grading_policy:

https://github.mit.edu/mitx/content-mit-805x/blob/edgeccx/policies/1T2015/grading_policy.json#L17-L28

Two assignment types are missing the min_count key. When we try to save the CCX schedule, we check the min_count to see if we need to update it due to assignments being removed from the CCX. If the key doesn't exist, it results in a 500:

Traceback (most recent call last):

 File "/edx/app/edxapp/venvs/edxapp/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 109, in get_response
   response = callback(request, *callback_args, **callback_kwargs)

 File "/edx/app/edxapp/venvs/edxapp/local/lib/python2.7/site-packages/django/utils/decorators.py", line 91, in _wrapped_view
   response = view_func(request, *args, **kwargs)

 File "/edx/app/edxapp/venvs/edxapp/local/lib/python2.7/site-packages/django/views/decorators/cache.py", line 75, in _cache_controlled
   response = viewfunc(request, *args, **kw)

 File "/edx/app/edxapp/edx-platform/lms/djangoapps/ccx/views.py", line 99, in wrapper
   return view(request, course, ccx)

 File "/edx/app/edxapp/edx-platform/lms/djangoapps/ccx/views.py", line 258, in save_ccx
   if count < section['min_count']:

KeyError: 'min_count'

We should protect against this. It's safe to assume that min_count = 0 when there is no value.