perf: target to es6, fixes #137

BREAKING CHANGE: ship to Node.js 8, the dist/liquid.cjs.js (main) nolonger
supports Node.js 6, other bundles are also provided via dist/liquid.esm.js, dist/liquid.js (ES5 umd) and liquid.min.js (minified ES5 umd)

src/liquid.ts

@@ -91,5 +91,4 @@ export class Liquid {
       self.renderFile(filePath, ctx, opts).then(html => cb(null, html), cb)
     }
   }
-  public static default = Liquid // compatible to import { Liquid } from 'liquidjs'
 }