Remove extraneous non-enumerable in internals.skeleton

[WesTyler]

Description

The non-enumerable property assignment in internals.skeleton was unneccesary and was triggering a bug in the V8 implementation of JSON.stringify in Node v4.x

Motivation and Context

Consumers of Felicity.entityFor that try to save new'd instances into elasticsearch in Node v4.5.0 would have the non-enumerable .schema property stringified into the documents erroneously.

const schema = Joi.object().keys({a: Joi.string(), b: Joi.object().keys({c: Joi.string()})})\
const Thing = Felicity.entityFor(schema)
const thing2 = new Thing({a: 'here', b: {c: 'here'}})

JSON.stringify(thing2);

// '{"a":"here","b":{"c":"here"}}'

JSON.stringify(thing2, undefined, undefined)
/*
'{"a":"here","b":{"c":"here"},"schema":{"isJoi":true,"_type":"object","_settings":null,"_valids":{"_set":[]},"_invalids":{"_set":[]},"_tests":[],"_refs":[],"_flags":{},"_description":null,"_unit":null,"_notes":[],"_tags":[],"_examples":[],"_meta":[],"_inner":{"children":[{"key":"a","schema":{"isJoi":true,"_type":"string","_settings":null,"_valids":{"_set":[]},"_invalids":{"_set":[""]},"_tests":[],"_refs":[],"_flags":{},"_description":null,"_unit":null,"_notes":[],"_tags":[],"_examples":[],"_meta":[],"_inner":{}}},{"key":"b","schema":{"isJoi":true,"_type":"object","_settings":null,"_valids":{"_set":[]},"_invalids":{"_set":[]},"_tests":[],"_refs":[],"_flags":{},"_description":null,"_unit":null,"_notes":[],"_tags":[],"_examples":[],"_meta":[],"_inner":{"children":[{"key":"c","schema":{"isJoi":true,"_type":"string","_settings":null,"_valids":{"_set":[]},"_invalids":{"_set":[""]},"_tests":[],"_refs":[],"_flags":{},"_description":null,"_unit":null,"_notes":[],"_tags":[],"_examples":[],"_meta":[],"_inner":{}}}],"renames":[],"dependencies":[],"patterns":[]}}}],"renames":[],"dependencies":[],"patterns":[]}}}'
*/

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue. you didn't modify existing tests)

Checklist:

• [x] I have read the CONTRIBUTING document.
• [x] My code follows the Hapi.js style guide.
• [x] I have updated the documentation as needed.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.