chore(travis): adds node 4.x to build pipeline

Relates to stalniy#1

Author: stalniy

.travis.yml

@@ -1,5 +1,6 @@
 language: node_js
 node_js:
+  - "4.0.0"
   - "6.9.0"
   - "8.1.3"
 

spec/mongoose-plugin.spec.js

@@ -41,12 +41,13 @@ describe('Mongoose Plugin', () => {
     expect(schema.query.accessibleBy).to.equal(schema.statics.accessibleBy)
   })
 
-  it('creates query which returns empty array or null for ability which does not have rules for specified action', async () => {
+  it('creates query which returns empty array or null for ability which does not have rules for specified action', () => {
     const query = schema.query.accessibleBy(new Ability())
-    const [items, item] = await Promise.all([query.exec(), query.findOne().exec()])
 
-    expect(items).to.be.an('array').that.is.empty
-    expect(item).not.to.exist
+    return Promise.all([query.exec(), query.findOne().exec()]).then(([items, item]) => {
+      expect(items).to.be.an('array').that.is.empty
+      expect(item).not.to.exist
+    })
   })
 
   describe('`accessibleBy` infterface', () => {