Why doesn't ruby find the 'dimensions' gem? - ruby

Question

I am using Jekyll for quite a while now to generate my blog (source). Today, without having changed anything in the generation process, I got
[...]
jekyll build --config _config_prod.yml
Configuration file: _config_prod.yml
/home/moose/GitHub/MartinThoma.github.io/_plugins/caption_tag.rb:23:in `require': cannot load such file -- dimensions (LoadError)
from /home/moose/GitHub/MartinThoma.github.io/_plugins/caption_tag.rb:23:in `<top (required)>'
from /var/lib/gems/2.1.0/gems/jekyll-2.5.3/lib/jekyll/plugin_manager.rb:74:in `require'
from /var/lib/gems/2.1.0/gems/jekyll-2.5.3/lib/jekyll/plugin_manager.rb:74:in `block (2 levels) in require_plugin_files'
from /var/lib/gems/2.1.0/gems/jekyll-2.5.3/lib/jekyll/plugin_manager.rb:73:in `each'
from /var/lib/gems/2.1.0/gems/jekyll-2.5.3/lib/jekyll/plugin_manager.rb:73:in `block in require_plugin_files'
from /var/lib/gems/2.1.0/gems/jekyll-2.5.3/lib/jekyll/plugin_manager.rb:72:in `each'
from /var/lib/gems/2.1.0/gems/jekyll-2.5.3/lib/jekyll/plugin_manager.rb:72:in `require_plugin_files'
from /var/lib/gems/2.1.0/gems/jekyll-2.5.3/lib/jekyll/plugin_manager.rb:18:in `conscientious_require'
from /var/lib/gems/2.1.0/gems/jekyll-2.5.3/lib/jekyll/site.rb:79:in `setup'
from /var/lib/gems/2.1.0/gems/jekyll-2.5.3/lib/jekyll/site.rb:41:in `initialize'
from /var/lib/gems/2.1.0/gems/jekyll-2.5.3/lib/jekyll/commands/build.rb:29:in `new'
from /var/lib/gems/2.1.0/gems/jekyll-2.5.3/lib/jekyll/commands/build.rb:29:in `process'
from /var/lib/gems/2.1.0/gems/jekyll-2.5.3/lib/jekyll/commands/build.rb:18:in `block (2 levels) in init_with_program'
from /usr/lib/ruby/vendor_ruby/mercenary/command.rb:220:in `call'
from /usr/lib/ruby/vendor_ruby/mercenary/command.rb:220:in `block in execute'
from /usr/lib/ruby/vendor_ruby/mercenary/command.rb:220:in `each'
from /usr/lib/ruby/vendor_ruby/mercenary/command.rb:220:in `execute'
from /usr/lib/ruby/vendor_ruby/mercenary/program.rb:42:in `go'
from /usr/lib/ruby/vendor_ruby/mercenary.rb:19:in `program'
from /var/lib/gems/2.1.0/gems/jekyll-2.5.3/bin/jekyll:20:in `<top (required)>'
from /usr/local/bin/jekyll:23:in `load'
from /usr/local/bin/jekyll:23:in `<main>'
Makefile:35: recipe for target 'deploy' failed
make: *** [deploy] Error 1
I am not a Ruby developer, but I guess the problem is require 'dimensions'? However, when I execute this line in irb it only shows => true.
What is the problem and how can I fix it?
Diagnostic information
$ jekyll --version
jekyll 2.5.3
$ head -1 `which irb`
#!/usr/bin/ruby2.1
$ head -1 `which jekyll`
#!/usr/bin/ruby2.1
Executing RUBY_VERSION in irb gives => "2.1.5".

Answer 1

The problem was fixed by adding gem 'dimensions' to the file Gemfile. It now looks like this:
source "https://rubygems.org"
# see https://github.com/paulrobertlloyd/paulrobertlloyd.com/blob/master/Gemfile
# Guard (automates development tasks)
# Jekyll (generates static files)
gem 'jekyll'
group :jekyll_plugins do
gem 'jekyll-paginate'
gem 'dimensions'
gem 'rmagick'
end
# Rake (manages build tasks)
gem 'rake'