Use cross-spawn for windows stats calc

Author: dy

package.json

@@ -111,6 +111,7 @@
     "brfs": "^1.4.3",
     "browserify": "^14.1.0",
     "browserify-transform-tools": "^1.7.0",
+    "cross-spawn": "^5.1.0",
     "deep-equal": "^1.0.1",
     "ecstatic": "^2.1.0",
     "eslint": "^3.17.1",

tasks/stats.js

@@ -1,6 +1,6 @@
 var path = require('path');
 var fs = require('fs');
-var spawn = require('child_process').spawn;
+var spawn = require('cross-spawn');
 
 var falafel = require('falafel');
 var gzipSize = require('gzip-size');