Update Rails to 4.1.8

Contains security fixes for:

* CVE-2014-3514 (4.1.5)
* CVE-2014-7818 (4.1.7)
* CVE-2014-7819 (4.1.7)
* CVE-2014-7829 (4.1.8)
Merge pull request #33 from opscode/nls/rails-update

Update Rails to 4.1.8