[#14507] Gemfile: awesome_nested_set: use github 2-1-stable branch

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

For Rails4 porting.

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

http://www.redmine.org/projects/redmine/repository/revisions/13145
[#14507] remove NestedSet patch from config/initializers/10-patches.rb

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

For Rails4 porting.