Make Interface indifferent to hash access

Fixes issue https://github.com/getsentry/raven-ruby/issues/123, whereby new Rails stack traces don't fit the old hash access. For backwards compatibility, I propose we just make the hash access indifferent.

For more context, see my comment here: https://github.com/getsentry/raven-ruby/issues/123#issuecomment-42764441

Using `Hashie::Extensions::Dash::IndifferentAccess` would be preferable, but this is just 6 days old. For now it's fine to use `Hashie::Extensions::IndifferentAccess`.