Update Hashie method name

The method name changed in the latest version of Hashie, causing the following exception:

```
WARN: super: no superclass method `assert_required_properties_set!' for #<Raven::StacktraceInterface::Frame:0x00000002d9fe98
WARN: /var/lib/gems/2.0.0/gems/sentry-raven-0.9.3/lib/raven/interfaces.rb:21:in `assert_required_properties_set!'
(...)
```

This is one of three possible solutions. The other two are:

1. Put a pessimistic version constraint on `hashie`.
2. Check for both method names before making a call.