fixing cache clear method

[shanbady]

Description (What does it do?)

Resolves an issue with mis-named cache clear method. from main.utils import clear_cache should be from main.utils import clear_search_cache

How can this be tested?

checkout this branch and run the affected commands:

python manage.py update_platforms
python manage.py update_offered_by
python manage.py update_departments_schools
python manage.py populate_featured_lists

They should succeed