Properly support symbol keys in table hashes

Re: #305

At a glance, the implementation in `ast/table.rb:383` just uses Hash's default value block to fetch missing symbol keys by string. A few more pieces are needed to make it truly blind... To avoid a dependency on activesupport gem, this pull request grabs a copy of HashWithIndifferentAccess and loads it in the Cucumber namespace when the original is not defined.