src: Inline util.promisify

Here we implement the feature without adding an extra
dependency.

src/parser.coffee

@@ -6,7 +6,6 @@ bom = require './bom'
 processors = require './processors'
 setImmediate = require('timers').setImmediate
 defaults = require('./defaults').defaults
-promisify = require 'util.promisify'
 
 # Underscore has a nice function for this, but we try to go without dependencies
 isEmpty = (thing) ->
@@ -255,7 +254,12 @@ class exports.Parser extends events
         throw err
 
   parseStringPromise: (str) =>
-    promisify(@parseString) str
+    new Promise (resolve, reject) =>
+      @parseString str, (err, value) =>
+        if err
+          reject err
+        else
+          resolve value
 
 exports.parseString = (str, a, b) ->
   # let's determine what we got as arguments

test/parser.test.coffee

@@ -5,11 +5,16 @@ util = require 'util'
 assert = require 'assert'
 path = require 'path'
 os = require 'os'
-promisify = require 'util.promisify'
 
 fileName = path.join __dirname, '/fixtures/sample.xml'
 
-readFilePromise = promisify fs.readFile
+readFilePromise = (fileName) ->
+  new Promise (resolve, reject) =>
+    fs.readFile fileName, (err, value) =>
+      if err
+        reject err
+      else
+        resolve value
 
 skeleton = (options, checks) ->
   (test) ->