fix `__bobthefish_vagrant_ids` when no id present

because `cat` with no arguments just hangs out at stdin waiting for you to type something.

fish_prompt.fish

@@ -198,7 +198,9 @@ function __bobthefish_prompt_vagrant -S -d 'Display Vagrant status'
 end
 
 function __bobthefish_vagrant_ids -S -d 'List Vagrant machine ids'
-  cat .vagrant/machines/**/id
+  for id in .vagrant/machines/**/id
+    cat $id
+  end
 end
 
 function __bobthefish_prompt_vagrant_vbox -S -d 'Display VirtualBox Vagrant status'