Changing readme example of how to load config from JSON

After reviewing the Grunt upgrade notes for 0.3 to 0.4 I can see they have removed:

`'<json:file.json>' `

in favour of

`grunt.file.readJSON('file.json')`

https://github.com/gruntjs/grunt/wiki/Upgrading-from-0.3-to-0.4

Have updated grunt s3 readme accordingly.