Stop relying on Hashie

TODO: add tests :-(

Questions:
- Is the `required` option really necessary? Of the failing tests, none
of them seem to be related to that.
- Just how necessary is having a hash with indifferent access? I
noticed that if I change the hash keys to all be strings, most of the
existing tests passed. In the Ruby world it’s better to use symbols for
this sort of data, so is it okay to update the tests to reference the
attributes as symbols?
remove the required option
get the existing tests passing

- store all attributes with a string key
- update attr_accessor to not clobber false values
- update event_spec.rb to include an attribute that’s nil
- update stack_trace.rb to use default values
add tests, fix a bug
use 1.8 hash syntax
backport `Base64.strict_encode64` for Ruby 1.8
Merge branch 'remove-hashie' of https://github.com/seanlinsley/raven-ruby into seanlinsley-remove-hashie
Remove duplicate abs_path accessor
Execute rake inside of bundler
Force longer timeout in CLI usage