Podspec update

* Delete the deprecated `documentation` attribute
  * It’s now automatically handled by [CocoaDocs](http://cocoadocs.org/docsets/Fingertips/0.3.0/)

* Change homepage to https instead of http
  * So that `pod spec lint` is happy

* Add `social_media_url` DSL directive
  * So that @CocoaPodsFeed can mention @Mapbox