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.
I think version specifier should be like this.
```
gem.add_dependency "hashie", "~> 3.1.0"
```

Because hashies this method name may change.
Version 3.0.0 of hashie has method assert_required_properties_set! .
And version 3.1.0 of hashie has method assert_required_attributes_set! .
In the future, newer version of hashie may has different name one.