Use proper license name

Recent versions of RubyGems complain that license must be one of those
listed at https://spdx.org/licenses.

rb/selenium-webdriver.gemspec

@@ -12,7 +12,7 @@ It aims to mimic the behaviour of a real user, and as such interacts with the
 HTML of the application.'
   s.summary = 'The next generation developer focused tool for automated testing of webapps'
   s.homepage = 'https://github.com/seleniumhq/selenium'
-  s.licenses = ['Apache']
+  s.license = 'Apache-2.0'
 
   s.required_rubygems_version = Gem::Requirement.new('> 1.3.1') if s.respond_to? :required_rubygems_version=
   s.required_ruby_version = Gem::Requirement.new('>= 2.0')