• Update to 3.1.5
• Fix LICENSE
• Add LICENSE_FILE

According to README.md, this software is distributed under the same terms as
ruby with 2 exceptions:

• lib/test/unit/diff.rb is a double license of the Ruby license and PSF license.
• lib/test-unit.rb is a dual license of the Ruby license and LGPLv2.1 or later.

To fit in our LICENSE framework, I changed LICENSE to GPLv2 and RUBY which cover
entire software.

Changes: https://github.com/test-unit/test-unit/blob/master/doc/text/news.md
Reference: https://github.com/test-unit/test-unit/blob/master/README.md#license