Bump djangorestframework from 3.9.0 to 3.9.1

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.9.0 to 3.9.1.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](encode/django-rest-framework@3.9.0...3.9.1)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

Pipfile

@@ -20,7 +20,7 @@ django-storages = "==1.7.1"
 django-taggit = "==0.23.0"
 django-toolbelt = "==0.0.1"
 django-treebeard = "==4.3"
-djangorestframework = "==3.9.0"
+djangorestframework = "==3.9.1"
 docutils = "==0.14"
 gunicorn = "==19.7.1"
 html5lib = "==1.0.1"

Pipfile.lock

@@ -1,11 +1,11 @@
+-i https://pypi.org/simple/
 beautifulsoup4==4.6.0
 boto3==1.9.91
 botocore==1.12.91
 certifi==2018.11.29
 chardet==3.0.4
 dj-database-url==0.5.0
 dj-static==0.0.6
-Django==2.1.11
 django-appconf==1.0.2
 django-compressor==2.2
 django-environ==0.4.5
@@ -15,17 +15,18 @@ django-storages==1.7.1
 django-taggit==0.23.0
 django-toolbelt==0.0.1
 django-treebeard==4.3
+django==2.1.5
 djangorestframework==3.9.1
 docutils==0.14
 draftjs-exporter==2.1.5
 gunicorn==19.7.1
 html5lib==1.0.1
 idna==2.8
 jmespath==0.9.3
-libsass==0.17.0
-Pillow==6.2.0
-psycopg2==2.7.7
+libsass==0.19.4
+pillow==5.4.1
 psycopg2-binary==2.7.7
+psycopg2==2.7.7
 python-dateutil==2.8.0
 python-dotenv==0.10.1
 pytz==2018.9
@@ -35,9 +36,9 @@ rjsmin==1.0.12
 s3transfer==0.2.0
 six==1.12.0
 static3==0.7.0
-Unidecode==1.0.23
-urllib3==1.24.2
+unidecode==1.0.23
+urllib3==1.24.1
 wagtail==2.4
 webencodings==0.5.1
 whitenoise==4.1.2
-Willow==1.1
+willow==1.1