replace pjax with external resouce via composer

We are now maintaining a fork of pjax:
https://github.com/yiisoft/jquery-pjax

Author: cebe

composer.json

@@ -74,6 +74,7 @@
 		"lib-pcre": "*",
 		"yiisoft/yii2-composer": "*",
 		"yiisoft/jquery": "~2.0 | ~1.10",
+		"yiisoft/jquery-pjax": "*",
 		"ezyang/htmlpurifier": "4.6.*",
 		"cebe/markdown": "0.9.*"
 	},