Bump django to v1.8.15
Bump django to v1.8.15

References:
[1] https://www.djangoproject.com/weblog/2016/sep/26/security-releases/
[2] https://docs.djangoproject.com/en/1.10/releases/1.8.15/
[3] https://github.com/edx/edx-platform/pull/13598

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