add license information to gemspec
add license information to gemspec

this way it can be used with rubygems.org API

bino98/omniauth-misoca-oauth2,intridea/omniauth-oauth2,txssseal/omniauth-oauth2,Cloverhound/omniauth-ciscospark,local-ch/omniauth-oauth2,k-tada/omniauth-oauth2,billy-ran-away/omniauth-oauth2