Upgrade Rails to 4.1.8 to address CVE-2014-7829

To address the vulnerability [CVE-2014-7829](https://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2014-7829) Rails 4.1.8 [has been released](http://weblog.rubyonrails.org/2014/11/17/Rails-3-2-21-4-0-12-and-4-1-8-have-been-released/).

I think that this patch is important for certain users (me for example) on Heroku or using anyway ```config.serve_static_assets = true```. Depending on the hosting it might be difficult to find a workaround to the vulnerability, so the upgrade is encouraged.