An error occured while installing eventmachine

Exec[nickserver_bundler_update] fails on canvasback (cdev webapp):

   - [canvasback] err: /Stage[main]/Site_nickserver/Exec[nickserver_bundler_update]/returns: change from notrun to 0 failed: /bin/bash -c "/usr/bin/bundle check || /usr/bin/bundle install --path vendor/bundle" returned 5 instead of one of [0] at /srv/leap/puppet/modules/site_nickserver/manifests/init.pp:91

trying to debug using the cmd that puppet runs for Exec[nickserver_bundler_update] shows this:

root@canvasback:/srv/leap/nickserver# sudo -u nickserver /bin/bash -c "/usr/bin/bundle check || /usr/bin/bundle install --path vendor/bundle"
Your Gemfile's dependencies could not be satisfied
Install missing gems with `bundle install`
Fetching gem metadata from https://rubygems.org/.........
Fetching gem metadata from https://rubygems.org/..
Using rake (10.1.0) 
Using addressable (2.3.5) 
Using cookiejar (0.3.0) 
Using safe_yaml (0.9.7) 
Using crack (0.4.1) 
Installing eventmachine (1.0.3) with native extensions 
Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.

        /usr/bin/ruby1.9.1 extconf.rb 
/usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- mkmf (LoadError)
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from extconf.rb:2:in `'


Gem files will remain installed in /srv/leap/nickserver/vendor/bundle/ruby/1.9.1/gems/eventmachine-1.0.3 for inspection.
Results logged to /srv/leap/nickserver/vendor/bundle/ruby/1.9.1/gems/eventmachine-1.0.3/ext/gem_make.out
An error occured while installing eventmachine (1.0.3), and Bundler cannot continue.
Make sure that `gem install eventmachine -v '1.0.3'` succeeds before bundling.

(from redmine: created on 2013-10-08, closed on 2013-10-11, relates #4080 (closed), relates #4090 (closed), relates #3960 (closed))