fix(Flow): Relax flowtype defs for upcoming mongoose declarations.

See screenshots from:
https://twitter.com/nodkz/status/901183624950710272

Author: nodkz

src/composeWithMongoose.js

@@ -16,7 +16,7 @@ import type {
 } from './definition';
 
 export function composeWithMongoose(
-  model: MongooseModelT,
+  model: Object, // MongooseModelT, TODO use Model from mongoose_v4.x.x definition when it will be public
   opts: TypeConverterOpts = {}
 ): TypeComposer {
   const name: string = (opts && opts.name) || model.modelName;

src/fieldsConverter.js

@@ -17,7 +17,7 @@ import typeStorage from './typeStorage';
 
 import type {
   MongooseModelT,
-  MongooseModelSchemaT,
+  // MongooseModelSchemaT,
   MongoosePseudoModelT,
   MongooseFieldT,
   MongooseFieldMapT,
@@ -187,7 +187,7 @@ export function convertModelToGraphQL(
 }
 
 export function convertSchemaToGraphQL(
-  schema: MongooseModelSchemaT,
+  schema: Object, // MongooseModelSchemaT, TODO use Model from mongoose_v4.x.x definition when it will be public
   typeName: string
 ): TypeComposer {
   if (!typeName) {