Fix version of Django to prevent surprises from new releases

requirements.txt

@@ -15,7 +15,7 @@ coreapi==2.3.3
 coreschema==0.0.4
 coverage==4.5.4
 DendroPy==4.4.0
-Django>=2.2.4
+Django==2.2.4
 django-cors-headers==3.0.2
 django-debug-toolbar==2.0
 django-extensions==2.1.9