Use DRF extensions 1.2.2 to fix test suite bug

See http://nedbatchelder.com/blog/201612/finding_test_coupling.html for
the complete saga.

requirements/edx/base.txt

@@ -42,7 +42,7 @@ django==1.8.17
 djangorestframework-jwt==1.8.0
 djangorestframework-oauth==1.1.0
 edx-ccx-keys==0.2.1
-edx-drf-extensions==1.2.1
+edx-drf-extensions==1.2.2
 edx-lint==0.4.3
 edx-django-oauth2-provider==1.1.4
 edx-django-sites-extensions==2.1.1