Add license to gemspec

Prior to this commit, the gemspec did not include the license, which
means that one can't use the rubygems API to automatically determine
licensing of vendored gems. This commit adds the licensing to the
gemspec.

Author: Jeff Weiss

mysql2.gemspec

@@ -4,6 +4,7 @@ Gem::Specification.new do |s|
   s.name = %q{mysql2}
   s.version = Mysql2::VERSION
   s.authors = ["Brian Lopez"]
+  s.license = "MIT"
   s.email = %q{[email protected]}
   s.extensions = ["ext/mysql2/extconf.rb"]
   s.homepage = %q{http://github.com/brianmario/mysql2}