Relax conflicting dependency json "<=  1.7.7"

I'm having dependency issues
```ruby
Bundler could not find compatible versions for gem "json":
  In Gemfile:
    chef (>= 11.8.0) ruby depends on
      json (<= 1.7.7, >= 1.4.4) ruby

    vagrant-berkshelf (~> 1.3) ruby depends on
      json (1.8.1)
```