Gunicorn dependency.

requirements.txt

@@ -13,4 +13,5 @@ mutagen==1.44.0
 validators==0.12.1
 simplejson==3.17.0
 accessify==0.3.1
-librosa==0.7.2
\ No newline at end of file
+librosa==0.7.2
+gunicorn=20.0.4
\ No newline at end of file