bump drf to 3.4.7

our middleware src/sentry/utils/linksign.py when request.POST is accessed,
django multipartparser is invoked and the underlying WSGIRequest buf is exhausted before it can get to DRF.
WSGIRequest._files is still accessible, but they aren't read back into the buf on the 2nd time.

according to encode/django-rest-framework#3951, the fix should be in drf 3.4.7.

requirements-base.txt

@@ -13,7 +13,7 @@ django-jsonfield>=0.9.13,<0.9.14
 django-picklefield>=0.3.0,<1.1.0
 django-sudo>=2.1.0,<3.0.0
 Django>=1.8,<1.9
-djangorestframework==3.3.0
+djangorestframework==3.4.7
 email-reply-parser>=0.2.0,<0.3.0
 enum34>=1.1.6,<1.2.0
 functools32>=3.2.3,<3.3