core: avoid locking in trap context

lib/vagrant/ui.rb

@@ -123,9 +123,12 @@ module Vagrant
           data[i].gsub!("\r", "\\r")
         end
 
-        @lock.synchronize do
+        # Avoid locks in a trap context introduced from Ruby 2.0
-          safe_puts("#{Time.now.utc.to_i},#{target},#{type},#{data.join(",")}")
+        Thread.new do
-        end
+          @lock.synchronize do
+            safe_puts("#{Time.now.utc.to_i},#{target},#{type},#{data.join(",")}")
+          end
+        end.join
       end
     end
 
@@ -331,7 +334,6 @@ module Vagrant
         if @ui.is_a?(Vagrant::UI::MachineReadable)
           return machine(type, message, { :target => target })
         end
-         
 
         # Otherwise, make sure to prefix every line properly
         lines.map do |line|