authtwitter: Change API endpoint to use https://api.twitter.com/ .

This is specified at: http://dev.twitter.com/pages/auth#at-twitter

Thanks to Andjelko Horvat for fixing this.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2774 44740490-163a-0410-bde0-09ae8108e29a