Remove mention to GPLv2.

README.md expresses "lib/nata2/mysqldumpslow.rb contains the modified mysqldumpslow, this is GPL2 license applies".
Indeed, they have same input and they work similarly, but they're different implimentation.
This is not a "derived work" specified under GPLv2.

Thus, you don't have to apply GPLv2 to lib/nata2/mysqldumpslow.rb.
Of course you *can* apply GPLv2 to that if you want, but if you want apply GPLv2 to that, you have to apply GPLv2 to whole nata2's license.

I'm very sorry that I misunderstood to MySQL FOSS License Exception and told to you, this is my fault.