Require validation dependencies when installing drf-yasg

requirements.txt

@@ -6,7 +6,7 @@ django-mptt>=0.9.0
 django-tables2>=1.19.0
 django-timezone-field>=2.0
 djangorestframework>=3.7.7
-drf-yasg>=1.4.4
+drf-yasg[validation]>=1.4.4
 graphviz>=0.8.2
 Markdown>=2.6.11
 natsort>=5.2.0