Update foodie gem and guide, removed aruba step extensions

Hi @radar,

Ive updated the foodie gem example to include the latest versions of most dependant gems, the most important update being aruba. The guide has been updated to reflect the change in the example gem code.

Aruba now includes step definitions for checking multi-line contents of a file, making the aruba step definition extensions obsolete (any one new to the guide would get a `Cucumber::Ambiguous match error`, like this guy: http://stackoverflow.com/questions/17767030/how-to-resolve-a-cucumberambiguous-match-error-while-testing-a-cli-in-aruba).

I also updated a few `paths/to/file_names.ext` to use the inline code syntax ("`") instead of underscores, as it was making them render incorrectly. Rubygems source was also updated to source 'https://rubygems.org' (fixed deprecation warning).