Remove references to test-unit

[ncreuschling]

Execute bundle exec cucumber --expand --format pretty --format json --out=build/logs/json/cucumber.json 
cannot load such file -- test/unit/assertions (LoadError)
/home/jenkins/workspace/afrikarma-typo3-uat-develop/features/support/env.rb:27:in `require'

due to (features/support/env.rb):

# TestUnit
require 'test/unit/assertions'
World(Test::Unit::Assertions)

[ncreuschling]

dkdeploy-cucumber/lib/dkdeploy/cucumber/step_definitions/images.rb has to be rewritten without test-unit dependency.