Fix crash of progress page - Use `weight` instead of `max_score`

FYI @chrisndodge - using max_score was making the progress page crash with:

```
Apr 25 12:18:59 ip-10-0-11-19 [service_variant=lms][django.request][env:sandbox] ERROR [ip-10-0-11-19 1121] [base.py:215] - Internal Server Error: /courses/Demo/DM01/2014/progress
Traceback (most recent call last):
File "/edx/app/edxapp/venvs/edxapp/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 111, in get_response
response = callback(request, *callback_args, **callback_kwargs)
File "/edx/app/edxapp/venvs/edxapp/local/lib/python2.7/site-packages/django/contrib/auth/decorators.py", line 20, in _wrapped_view
return 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/venvs/edxapp/local/lib/python2.7/site-packages/django/db/transaction.py", line 224, in inner
return func(*args, **kwargs)
File "/edx/app/edxapp/edx-platform/lms/djangoapps/courseware/views.py", line 637, in progress
return _progress(request, course_id, student_id)
File "/edx/app/edxapp/edx-platform/lms/djangoapps/courseware/views.py", line 667, in _progress
courseware_summary = grades.progress_summary(student, request, course)
File "/edx/app/edxapp/venvs/edxapp/local/lib/python2.7/site-packages/django/db/transaction.py", line 224, in inner
return func(*args, **kwargs)
File "/edx/app/edxapp/edx-platform/lms/djangoapps/courseware/grades.py", line 315, in progress_summary
return _progress_summary(student, request, course)
File "/edx/app/edxapp/edx-platform/lms/djangoapps/courseware/grades.py", line 378, in _progress_summary
course_id, student, module_descriptor, module_creator, scores_cache=submissions_scores
File "/edx/app/edxapp/edx-platform/lms/djangoapps/courseware/grades.py", line 471, in get_score
total = problem.max_score()
TypeError: 'int' object is not callable
```