Change format of field 'engines'

[mkavidas]

Fixed the formatting so that package.json can pass the package.json validator .

The previous version was failing with the error: { "valid": false, "errors": [ "Type for field engines, was expected to be object, not object" ], "warnings": [ "Missing recommended field: keywords", "Missing recommended field: contributors" ], "recommendations": [ "Missing optional field: homepage" ] }