fix travis build status image src of release/3.0 branch
refactor session store initializer
set the path attribute of the session cookie when a relative url root is set
config can contain an empty string. use presence to convert that to nil. pass it directly to :path. rails handles the nil case with its default like before.
Merge pull request #1120 from marutosi/finn-rel-3.0-readme-src

fix travis build status image src of release/3.0 branch
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