Upgrade django to 1.11.15 for security update
Upgrade django to 1.11.15 for security update

There is a vulnerability of phishing in django 1.11.11 announced by
django community.To avoid it,upgrade current django version to 1.11.15
to overcome that vulnerability.

LEARNER-6034

edx/credentials,edx/credentials,edx/credentials,edx/credentials