Fix rake test fails with mocha > 2.1.0 #72

Using Mocha::Mock.new is not supported in mocha, and
mock() does not work in a subclass.
As we just need a stub for File:Statt an no mock, we
just create an Object (instead of a mock) and stub it.
This commit fixes #72

test/common.rb

@@ -87,7 +87,8 @@ def name
     end
 
     def stub!
-      stat = Mocha::Mock.new('file::stat')
+      # Stub for File::Stat
+      stat = Object.new
       stat.stubs(size: contents.length, mode: mode, mtime: mtime, atime: atime, directory?: false)
 
       File.stubs(:stat).with(path).returns(stat)
@@ -128,7 +129,8 @@ def directory(name, *args)
     def stub!
       Dir.stubs(:mkdir).with { |*a| a.first == path }
 
-      stat = Mocha::Mock.new('file::stat')
+      # Stub for File::Stat
+      stat = Object.new
       stat.stubs(size: 1024, mode: mode, mtime: mtime, atime: atime, directory?: true)
 
       File.stubs(:stat).with(path).returns(stat)