Update gulpfile to work in Windows

Gulp was trying to call bundle.bat but that doesn't work
in Linux.

gulpfile.js

@@ -11,7 +11,7 @@ var messages = {
  */
 gulp.task('jekyll-build', function (done) {
     browserSync.notify(messages.jekyllBuild);
-    return cp.spawn('bundle.bat', ['exec','jekyll','build'], {stdio: 'inherit'})
+    return cp.spawn('bundle', ['exec','jekyll','build'], {stdio: 'inherit'})
         .on('close', done);
 });