Bump gunicorn from 19.4.5 to 19.5.0

Bumps [gunicorn](https://github.com/benoitc/gunicorn) from 19.4.5 to 19.5.0.
- [Release notes](https://github.com/benoitc/gunicorn/releases)
- [Commits](benoitc/gunicorn@19.4.5...19.5.0)

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

Author: dependabot[bot]

requirements.txt

@@ -1,6 +1,6 @@
 dj-database-url==0.4.1
 Django==1.10
-gunicorn==19.4.5
+gunicorn==19.5.0
 model-mommy==1.3.1
 psycopg2-binary==2.8.5
 whitenoise==3.2