Sync BSD-2-Clause license

[syleeeee]

I found that the license is different from ruby/ruby.
https://github.com/ruby/ruby/blob/master/BSDL

Which one is supposed to be?

[kou]

Ruby is dual license of 2-clause BSDL and its original license you removed.
We must not remove the original license part.

See also the part you didn't remove:

You can redistribute it and/or modify it under either the terms of the
2-clause BSDL (see the file BSDL), or the conditions below:

[syleeeee]

Oh, I didn't see or. Let me close this PR. Thank you.