Update pygments from 2.2.0 to 2.3.1

Author: pyup-bot

requirements.txt

@@ -14,7 +14,7 @@ future==0.17.1
 jsmin==2.2.2
 Pillow==6.0.0
 psycopg2==2.8.2
-Pygments==2.2.0
+Pygments==2.3.1
 python-decouple==3.1
 redis==3.2.0
 gunicorn==19.9.0