update django-oauth2-provide version

TNL-3820

requirements.txt

@@ -2,5 +2,5 @@ django>=1.8,<1.9
 djangorestframework>=3.1,<3.2
 enum34==1.0.4
 lxml==3.3.6
--e git+https://github.com/edx/django-oauth2-provider.git@0.2.7-fork-edx-5#egg=django-oauth2-provider
+-e git+https://github.com/edx/django-oauth2-provider.git@0.2.7-fork-edx-6a#egg=django-oauth2-provider==0.2.7-fork-edx-6
 -e git+https://github.com/edx/django-rest-framework-oauth.git@f0b503fda8c254a38f97fef802ded4f5fe367f7a#egg=djangorestframework-oauth