make compatible with latest stable redmine 2.3.1

Redmine 2.3.1 does allow 
gem "jquery-rails", "~> 2.0.2"

see http://www.redmine.org/projects/redmine/repository/revisions/11953/entry/branches/2.3-stable/Gemfile#L4
Merge pull request #1 from cforce/patch-1

make compatible with latest stable redmine 2.3.1