JSON parsing tweak

Switched to using 'multi_json'in order to be a little more flexible when
parsing JSON.
Gem dependency updates

Locked down the version of 'gherkin' being used in order to not lose
Ruby 1.8.7 support and added explicit dependence on the 'json' and
'multi_json' gems in order to guarantee their inclusion rather than rely
on their inclusion via other dependencies.
Code 'require' updates

 Added explicit 'require' statements for external code in order to
 guarantee its inclusion rather than rely on inclusion via other
 dependencies.
Quieter test output

Adjusted the Rake task for the gem's Cucumber tests so that it does not
output the source code of the tests when they are run.