update raven to version 5.31.0 to avoid runtime error caused by SentryMiddleware when running django wsgi with gunicorn in python 3.5

CONTRIBUTORS.rst

@@ -58,6 +58,7 @@ Listed in alphabetical order.
   Benjamin Abel
   Bo Lopker                `@blopker`_
   Bouke Haarsma
+  Brent Payne              `@brentpayne`_               @brentpayne
   Burhan Khalid            `@burhan`_                   @burhan
   Catherine Devlin         `@catherinedevlin`_
   Cédric Gaspoz            `@cgaspoz`_

{{cookiecutter.project_slug}}/requirements/production.txt

@@ -29,7 +29,7 @@ django-anymail==0.5
 {% if cookiecutter.use_sentry_for_error_reporting == "y" -%}
 # Raven is the Sentry client
 # --------------------------
-raven==5.30.0
+raven==5.31.0
 {%- endif %}
 
 {% if cookiecutter.use_opbeat == "y" -%}