refactor: es6 building and linting

test/options/strict.js

@@ -4,8 +4,8 @@ const Liquid = require('../../src')
 chai.use(require('chai-as-promised'))
 
 describe('strict options', function () {
-  var engine
-  var ctx = {}
+  let engine
+  let ctx = {}
   beforeEach(function () {
     engine = Liquid({
       root: '/root/',
@@ -17,16 +17,16 @@ describe('strict options', function () {
       .eventually.equal('beforeafter')
   })
   it('should throw when strict_variables true', function () {
-    var tpl = engine.parse('before{{notdefined}}after')
-    var opts = {
+    let tpl = engine.parse('before{{notdefined}}after')
+    let opts = {
       strict_variables: true
     }
     return expect(engine.render(tpl, ctx, opts)).to
       .be.rejectedWith(/undefined variable: notdefined/)
   })
   it('should pass strict_variables to render by parseAndRender', function () {
-    var html = 'before{{notdefined}}after'
-    var opts = {
+    let html = 'before{{notdefined}}after'
+    let opts = {
       strict_variables: true
     }
     return expect(engine.parseAndRender(html, ctx, opts)).to