Remove json gem version restriction

Hi guys --

We were unable to use this gem without making the following modifications:
- Removing the pessimistic version constraint on the json gem.  It was locked to 1.5.x.  It appears as though 4eb1f5612cf281a6d699a33d315a5e981650beca attempted to do this by modifying `r_hapi.gemspec` directly, instead of using Jeweler to regenerate it.

- Bumping Jeweler version to remove the Bundler 1.0.x version constraint.

The newly-regenerated gemspec file looks like it has some new things because of the Jeweler version update, as well.