Locking Hashie to < Version 3.0.

Hey,
Hashie released version 3.0 on Tuesday. It breaks sentry.

Issue is on line 18 ```/lib/raven/interfaces/stack_trace.rb```.

New version of Hashie is making each key in the data hash a symbol instead of a string.

So data['frame'] = nil
But data[:frame] has correct data.

Stack trace.

I've locked hashie to 2.1.1 in all my projects to fix for now.

```
undefined method `map' for nil:NilClass 
/home/rails/mc/releases/20140604184757/vendor/bundle/ruby/1.9.1/gems/sentry-raven-0.8.0/lib/raven/interfaces/stack_trace.rb:18:in `to_hash' 
/home/rails/mc/releases/20140604184757/vendor/bundle/ruby/1.9.1/gems/sentry-raven-0.8.0/lib/raven/event.rb:116:in `block in to_hash' 
/home/rails/mc/releases/20140604184757/vendor/bundle/ruby/1.9.1/gems/sentry-raven-0.8.0/lib/raven/event.rb:115:in `each_pair' 
/home/rails/mc/releases/20140604184757/vendor/bundle/ruby/1.9.1/gems/sentry-raven-0.8.0/lib/raven/event.rb:115:in `to_hash' 
/home/rails/mc/releases/20140604184757/vendor/bundle/ruby/1.9.1/gems/sentry-raven-0.8.0/lib/raven/client.rb:50:in `encode' 
/home/rails/mc/releases/20140604184757/vendor/bundle/ruby/1.9.1/gems/sentry-raven-0.8.0/lib/raven/client.rb:34:in `send' 
/home/rails/mc/releases/20140604184757/vendor/bundle/ruby/1.9.1/gems/sentry-raven-0.8.0/lib/raven/base.rb:69:in `send' 
/home/rails/mc/releases/20140604184757/vendor/bundle/ruby/1.9.1/gems/sentry-raven-0.8.0/lib/raven/base.rb:107:in `block in capture_exception' 
/home/rails/mc/releases/20140604184757/vendor/bundle/ruby/1.9.1/gems/sentry-raven-0.8.0/lib/raven/base.rb:128:in `send_or_skip' 
/home/rails/mc/releases/20140604184757/vendor/bundle/ruby/1.9.1/gems/sentry-raven-0.8.0/lib/raven/base.rb:101:in `capture_exception' 
```