Fix 1.8.7 compatibility issue

lib/vagrant/config/vm/provisioner.rb

@@ -18,12 +18,17 @@ module Vagrant
 
         # Configures the provisioner if it can (if it is valid).
         def configure(options=nil, &block)
+          # We don't want ancestors to be searched. This is the default in 1.8,
+          # but not in 1.9, hence this hackery.
+          const_args = ["Config"]
+          const_args << false if RUBY_VERSION >= "1.9"
+
           # We assume that every provisioner has a `Config` class beneath
           # it for configuring.
-          return if !@provisioner || !@provisioner.const_defined?("Config", false)
+          return if !@provisioner || !@provisioner.const_defined?(*const_args)
 
           # Instantiate the config class and configure it
-          @config = @provisioner.const_get("Config", false).new
+          @config = @provisioner.const_get(*const_args).new
           block.call(@config) if block
 
           # TODO: Deal with the options hash