Update dependencies

Hi there,

I updated a couple dependencies in the gemspec so that the blitz gem is usable in modern developments.

* old json dependency had issues with capistrano
* old rspec didn't allow for using new rspec if the gem was added as a test dependency in a project using a newer rspec

I would have bumped the version number, but I was not sure about how you guys do it - manually or not (the comment at the beginning of the gem suggest not manually)