allOf (value, schema, opts) {
    let allErrors = []
    schema.allOf.forEach(function (_schema) {
      allErrors = allErrors.concat(validate(value, _schema, opts) || [])
    })
    return allErrors.length ? allErrors : undefined
  }
Validates the provided value against all schemas defined in the Schemas `allOf` keyword.
The instance is valid against if and only if it is valid against all the schemas declared in the Schema's value.

The value of this keyword MUST be an array. This array MUST have at least one element.
Each element of this array MUST be a valid JSON Schema.

see http://json-schema.org/latest/json-schema-validation.html#anchor82

@name Schema.validationKeywords.allOf
@method
@param {*} value Value to be validated.
@param {object} schema Schema containing the `allOf` keyword.
@param {object} [opts] Configuration options.
@returns {(array|undefined)} Array of errors or `undefined` if valid.