Fix license identifier in the ruby-fizzbuzz.gemspec file.
Fix license identifier in the ruby-fizzbuzz.gemspec file.

This commit fixes the license identifier as per the warning message
from RubyGems and makes it compliant with the format available at
http://spdx.org/licenses.

Signed-off-by: Krzysztof Wilczynski <5f1c0be89013f8fde969a8dcb2fa1d522e94ee00@linux.com>

kwilczynski/ruby-fizzbuzz,kwilczynski/ruby-fizzbuzz,kwilczynski/ruby-fizzbuzz