Unpin django-storages in edx-platform

[KristinAoki]

There is a pin for django-storages<1.14.4. A recent upgrade to django-storages from 1.14.3 to 1.14.4 broke the ability to import a course. The course was able to upload but after uploading the file was not able to be found when the import moved to the "Unpacking" step. Based on community discovery, the culprit are these two lines added in 1.14.4 that make file_exists_in_storage function always return False, as the default value of AWS_S3_FILE_OVERWRITE is True.