fix a bug in `rake spec' which it would raise an ArgumentError when use "it" without block

A following spec will raise an ArgumentError.

	describe "Application 'test'" do
	  it "has one window"
	end

`instance_eval' will pass a receiver object into block.
When use a block which created by lambda, it should accept a receiver object as block parameter.

	b = lambda { self.upcase }
	"test".instance_eval(&b) #=> wrong number of arguments (1 for 0) (ArgumentError)

lib/motion/spec.rb

@@ -490,7 +490,7 @@ module Bacon
 
     def it(description, &block)
       return  unless description =~ RestrictName
-      block ||= lambda { should.flunk "not implemented" }
+      block ||= proc { should.flunk "not implemented" }
       Counter[:specifications] += 1
       @specifications << Specification.new(self, description, block, @before, @after)
     end