Ensure guest names are string when filtering

Prior to this commit, if a guest name was given as a symbol, the
filter_triggers method would fail to properly match it with the only_on
option, as it is not a valid type to the #String.match method. This
commit fixes that by converting the parameter to a string so that it can
be properly matched on the guest.

lib/vagrant/plugin/v2/trigger.rb

@@ -110,7 +110,7 @@ module Vagrant
             match = false
             if trigger.only_on
               trigger.only_on.each do |o|
-                if o.match(guest_name)
+                if o.match(guest_name.to_s)
                   # trigger matches on current guest, so we're fine to use it
                   match = true
                   break

test/unit/vagrant/plugin/v2/trigger_test.rb

@@ -74,7 +74,7 @@ describe Vagrant::Plugin::V2::Trigger do
 
       after_triggers = triggers.after_triggers
       expect(after_triggers.size).to eq(3)
-      subject.send(:filter_triggers, after_triggers, "ubuntu", :action)
+      subject.send(:filter_triggers, after_triggers, :ubuntu, :action)
       expect(after_triggers.size).to eq(2)
     end