implement delete to also delete the string key
Merge remote-tracking branch 'origin/master' into string-key-fix-from-zendesk

To pick up Travis CI changes

Fixes #3
Bump version to 0.0.3

TODO
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Upload to https://rubygems.org/gems/rails_4_session_flash_backport

CC: @lparry
Merge pull request #4 from envato/string-key-fix-from-zendesk

Use Zendesk's fix for delete with symbol keys in rails 2