Rubber installed but isn't usable? - ruby-on-rails

Question

I think there's something wrong with my setup.
I am trying to install Rubber following this guide:
https://github.com/rubber/rubber/wiki/Quick-Start
It appears that I installed rubber correctly...
$ sudo gem install rubber
$ Successfully installed rubber-2.7.4
$ 1 gem installed
But I cannot invoke it...
$ rubber vulcanize complete_passenger_postgresql
$ /usr/lib/ruby/1.9.1/rubygems/dependency.rb:247:in `to_specs': Could not find rubber (>= 0) amongst [actionmailer-4.0.3, actionmailer-4.0.2
I am confused. I use rvm and every gem should be stored inside ~/.rvm/gems. Why is the /usr/lib/ruby directory being involved at all?
Any help would be great
More info on my environment:
gem environment
RubyGems Environment:
- RUBYGEMS VERSION: 2.2.2
- RUBY VERSION: 2.1.0 (2013-12-25 patchlevel 0) [i686-linux]
- INSTALLATION DIRECTORY: /home/evan/.rvm/gems/ruby-2.1.0
- RUBY EXECUTABLE: /home/evan/.rvm/rubies/ruby-2.1.0/bin/ruby
- EXECUTABLE DIRECTORY: /home/evan/.rvm/gems/ruby-2.1.0/bin
- SPEC CACHE DIRECTORY: /home/evan/.gem/specs
- RUBYGEMS PLATFORMS:
- ruby
- x86-linux
- GEM PATHS:
- /home/evan/.rvm/gems/ruby-2.1.0
- /home/evan/.rvm/gems/ruby-2.1.0#global
- GEM CONFIGURATION:
- :update_sources => true
- :verbose => true
- :backtrace => false
- :bulk_threshold => 1000
- REMOTE SOURCES:
- https://rubygems.org/
- SHELL PATH:
- /home/evan/.opam/system/bin
- /home/evan/.rvm/gems/ruby-2.1.0/bin
- /home/evan/.rvm/gems/ruby-2.1.0#global/bin
- /home/evan/.rvm/rubies/ruby-2.1.0/bin
- /home/evan/Apps/AWS-ElasticBeanstalk-CLI-2.6.0/eb/linux/python2.7
- /usr/local/heroku/bin
- /usr/lib/lightdm/lightdm
- /usr/local/sbin
- /usr/local/bin
- /usr/sbin
- /usr/bin
- /sbin
- /bin
- /usr/games
- /home/evan/Apps/sketch_tar/sketch-1.6.4/sketch-frontend
- /home/evan/Apps/yices-1.0.39/bin
- /home/evan/Apps/arduino-1.0.5
- /home/evan/Apps/cil/cil-1.7.3/bin
- /home/evan/Apps/intellej/idea-IC-133.696/bin
- /home/evan/Apps/play-2.2.1
- /home/evan/Apps/scala-2.10.3/bin
- /home/evan/Apps/apache-maven-3.0.5/bin
- /home/evan/Apps/sublime/sublime2
- /home/evan/.rvm/bin

Answer 1

I got it!!
The problem is I was using rvm but I still prefixed my gem with sudo
i.e.
gem install ...
and
sudo gem install ...
has completely different environments. See this post:
Why do "gem" and "sudo gem" have different installation directories?
I was installing via "sudo gem install" but I was invoking the rubber command without sudo.