Update to latest `json` (1.8.0) gem

In certain environments, `json` gem install fails.

For me, this was happening with Ruby 2.0 on a Vagrant VM.

For reference on this bug, see:

  https://github.com/tscolari/mobylette/pull/30

Simply depending on `json` 1.8.x instead of 1.6.x fixed the issue for me.