Redmine

When trying to install plugins, instructions often say: run bunder rake and so on.
When doing so, I always get:

Redmine requires Bundler 1.5.0 or higher (you're using 1.3.5).
Please update with 'gem update bundler'.

I have tried do compile Ruby from source and so on, but with no success. How can I fix this ?

Also tried:

root@ubuntu14-redmine:/home/redmine/redmine/bin# gem update bundler
Updating installed gems
Updating bundler
Fetching: bundler-1.10.6.gem (100%)
Successfully installed bundler-1.10.6
Gems updated: bundler
Installing ri documentation for bundler-1.10.6...
Installing RDoc documentation for bundler-1.10.6...
root@ubuntu14-redmine:/home/redmine/redmine/bin#
root@ubuntu14-redmine:/home/redmine/redmine/bin#
root@ubuntu14-redmine:/home/redmine/redmine/bin#
root@ubuntu14-redmine:/home/redmine/redmine/bin# bundle
Redmine requires Bundler 1.5.0 or higher (you're using 1.3.5).
Please update with 'gem update bundler'.
root@ubuntu14-redmine:/home/redmine/redmine/bin# gem update bundler
Updating installed gems
Nothing to update
root@ubuntu14-redmine:/home/redmine/redmine/bin#

Versjon of Remine and Ruby:

Environment:
Redmine version 3.1.1.stable.14867
Ruby version 2.1.2-p95 (2014-05-08) [x86_64-linux]
Rails version 4.2.4
Environment production
Database adapter Mysql2
SCM:
Subversion 1.8.8
Git 1.9.1
Filesystem
Redmine plugins:
no plugin installed