[#14507] Rails4: Gemfile: awesome_nested_set: use gem

https://www.openproject.org/work_packages/14507

There is no answer on #1864.

For Rails4 porting.

https://github.com/opf/openproject/commit/2e8057223cccb7743d8f018ccd37403c645751f1

http://www.redmine.org/projects/redmine/repository/revisions/13145
http://www.redmine.org/projects/redmine/repository/revisions/13292