Update example for clearing context
Update example for clearing context

In ee77e94dfd8b86a7375414fa40f72d1a72e74bd0 the `clear!` method was
changed from an instance method to a class method.

`Raven.context` returns an instance, so Raven.context.clear! does not
work.

sunkencity/raven-ruby,lsb/raven-ruby,timcheadle/raven-ruby,if1live/raven-ruby,spredfast/raven-ruby,alyssa/raven-ruby,ShakrMedia/raven-ruby,zanker/raven-ruby,looker/raven-ruby,jakecraige/raven-ruby,nateberkopec/raven-ruby,getsentry/raven-ruby