Fixed notify_change issue.(clipboard chain broken)

# steps
####  1.Run this code.  

test1.rb
```ruby
require 'win32/clipboard'

Win32::Clipboard.notify_change{puts "foo"}
```

#### 2. Run this code.  

test2.rb:
```ruby
require 'win32/clipboard'

Win32::Clipboard.notify_change{puts "bar"}
```

#### 3. Kill a process of "ruby test2.rb" and then copy something. 

# problem
The result should be "foo", but nothing.


I'm using the following:

Windows 7 (x64)
Ruby 2.0.0p353 (2013-11-22)[x64-mingw32]
win32/clipboard 0.6.2