minor: a few small gemfile tweaks
minor: a few small gemfile tweaks

- removing redundant json gem in dev
- making guard notifications windows and linux friendly

ukblewis/bson-ruby,johnsimon/bson-ruby,estolfo/bson-ruby,estolfo/bson-ruby,ukblewis/bson-ruby,mongodb/bson-ruby,agis-/bson-ruby,agis-/bson-ruby,hudon/bson-ruby,mongodb/bson-ruby,mongodb/bson-ruby,agis-/bson-ruby,mongodb/bson-ruby,hudon/bson-ruby,ukblewis/bson-ruby,johnsimon/bson-ruby,ukblewis/bson-ruby,johnsimon/bson-ruby,hudon/bson-ruby,estolfo/bson-ruby,estolfo/bson-ruby