oboe.gemspec doesn't specify Ruby 1.8 json dependency

At least under Ruby 1.8 (possibly others), the oboe gem depends upon the json gem.  This should be specified in the oboe.gemspec file.

Found during a puppetmaster instrumentation test in a clean environment.

![screen shot 2013-12-10 at 7 54 02 pm](https://f.cloud.github.com/assets/395132/1717358/3bc9405a-61ce-11e3-804d-09f54208525d.png)