[release/4.0] Fix CVE-2014-7818 and CVE-2014-7819

Addresses CVE-2014-7818 and CVE-2014-7819.

* **CVE-2014-7818** There is an information leak vulnerability in Action Pack.
  see: https://groups.google.com/forum/#!topic/rubyonrails-security/dCp7duBiQgo

* **CVE-2014-7819** There is an information leak vulnerability in Sprockets.
  see: https://groups.google.com/forum/#!topic/rubyonrails-security/wQBeGXqGs3E