merge !118

resolves #136 fix Error.call conflict when Opal is loaded

gulp.d/tasks/build-preview-pages.js

 'use strict'
 
+// NOTE remove patch after upgrading from asciidoctor.js to @asciidoctor/core
+Error.call = (self, ...args) => {
+  const err = new Error(...args)
+  return Object.assign(self, { message: err.message, stack: err.stack })
+}
+
 const asciidoctor = require('asciidoctor.js')()
 const fs = require('fs-extra')
 const handlebars = require('handlebars')