Add license to gemspec.
Add license to gemspec.

Closes #305
kristjan/httsoiree,sferik/httparty,rayzhng/httparty,rayzhng/httparty,charliesome/httpserious,ffleming/httparty,landonwilkins/httparty,jnunemaker/httparty,sferik/httparty,landonwilkins/httparty,supremebeing7/httparty,charliesome/httpserious,supremebeing7/httparty,jpb/httparty,RobinsonKameron/httparty,killerwails/httparty,lstoll/httpserious,wakermahmud/httparty,jnunemaker/httparty,eibay/httparty,jayjlawrence/httparty,jpb/httparty,kristjan/httsoiree,RobinsonKameron/httparty,supremebeing7/httparty,tokafish/httparty,lstoll/httpserious,charliesome/httpserious,jordanpagewhite/httparty,eibay/httparty,bob-frost/httparty,sideci-sample/sideci-sample-httparty,jnunemaker/httparty,landonwilkins/httparty,ffleming/httparty,jpb/httparty,tokafish/httparty,killerwails/httparty,tokafish/httparty,bob-frost/httparty,jayjlawrence/httparty,killerwails/httparty,jordanpagewhite/httparty,rayzhng/httparty,MichaelWiss/httparty,wakermahmud/httparty,wakermahmud/httparty,dpatlut/httparty,sideci-sample/sideci-sample-httparty,RobinsonKameron/httparty,lstoll/httpserious,jordanpagewhite/httparty,dpatlut/httparty,bob-frost/httparty,MichaelWiss/httparty,ffleming/httparty,kristjan/httsoiree,dpatlut/httparty,MichaelWiss/httparty,jayjlawrence/httparty,eibay/httparty