Gemfile.lock: Update nokogiri to fix security issues

See:
* https://groups.google.com/forum/#!topic/ruby-security-ann/DeJpjTAg1FA
* https://github.com/sparklemotion/nokogiri/blob/master/CHANGELOG.rdoc#1511--2013-12-14
Gemfile.lock: Update i18n to fix security issue

See
* https://groups.google.com/forum/#!topic/ruby-security-ann/pLrh6DUw998
* https://github.com/svenfuchs/i18n/commits/61f975bd862e2ce3a9f77fb5afdf11831fbcbb22
Merge pull request #1172 from opf/fix/i18n_and_nokogiri_vulnerabilities

Fix i18n and nokogiri vulnerabilities
infer content type from contents of file instead of its file extension
* defaults to application/binary as its more secure than octet-stream
* set nosniff header to instruct browsers not to guess content type