update rails to 4.1.8

Update Rails to version 4.1.8 to address [CVE-2014-7829] "Arbitrary
file existence disclosure in Action Pack".  More details here:
http://seclists.org/oss-sec/2014/q4/718
http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2014-7829
Merge pull request #106 from NYULibraries/chore/update-rails-CVE-2104-7829

update rails to 4.1.8 to address security concern