Fix parameterise deprecation warning

DEPRECATION WARNING: Passing the separator argument as a positional
parameter is deprecated and will soon be removed. Use `separator: '_'`
instead. (called from followable_translation_key at
/home/travis/build/consul/consul/app/controllers/follows_controller.rb:2
5)

app/controllers/follows_controller.rb

@@ -22,7 +22,7 @@ class FollowsController < ApplicationController
   end
 
   def followable_translation_key(followable)
-    followable.class.name.parameterize("_")
+    followable.class.name.parameterize(separator: "_")
   end
 
 end