Added version specifier to environment docs

The version specifier is required in Cher server 11. Updated the environment
docs to reflect this.

I was upgrading my Chef server from 10 to 11 and was having trouble running ``knife environment from file environments/myenv.json``. Turns out that this was happening when I had cookbooks in ``cookbook_versions`` without a version specifier. I just happened to notice that the ruby DSL for environment files had the version specifier included in the docs, but the JSON area did not. This update makes them consistent.