Use is-buffer instead of Buffer.isBuffer

The is-buffer module checks if an object is a Buffer without causing
Webpack or Browserify to include the whole Buffer module in the bundle.

lib/utils.js

@@ -1,6 +1,7 @@
 'use strict';
 
 var bind = require('./helpers/bind');
+var isBuffer = require('is-buffer');
 
 /*global toString:true*/
 
@@ -18,16 +19,6 @@ function isArray(val) {
   return toString.call(val) === '[object Array]';
 }
 
-/**
- * Determine if a value is a Node Buffer
- *
- * @param {Object} val The value to test
- * @returns {boolean} True if value is a Node Buffer, otherwise false
- */
-function isBuffer(val) {
-  return ((typeof Buffer !== 'undefined') && (Buffer.isBuffer) && (Buffer.isBuffer(val)));
-}
-
 /**
  * Determine if a value is an ArrayBuffer
  *