added license to .gemspec

Author: kzk

fluent-logger.gemspec

@@ -27,6 +27,7 @@ EOF
   gem.test_files    = `git ls-files -- {test,spec,features}/*`.split("\n")
   gem.executables   = `git ls-files -- bin/*`.split("\n").map{ |f| File.basename(f) }
   gem.require_paths = ['lib']
+  gem.license       = "Apache-2.0"
 
   gem.add_dependency "msgpack", ">= 1.0.0", "< 2"
   gem.add_development_dependency 'rake', '>= 0.9.2'