chore: react-docgen@5 is not supported, do not test it

Author: ZauberNerd

.tav.yml

@@ -13,5 +13,5 @@ node-6-7:
 node-8-x:
   name: react-docgen
   node: '>= 8.0.0'
-  versions: '>= 3.0.0 <= 6.0.0 || >= 5.0.0-beta.1'
+  versions: '>= 3.0.0 < 5.0.0 || 5.0.0-beta.1'
   commands: npm run test:unit