The CI job seems to install gems all the time instead of just checking that they are present. 

It needs to install them, because they are not present. The job runs a 'rake clean[all]' first which in turn actually does remove all gems. So the behaviour is intentional and is supposed to ensure that the gem environment and dependencies is working. Question is whether it realyl hurts, that the CI build is 2:30 min. IMO not.

If we decide, however, that we want to go back to something faster we can just run 'rake clean' and keep the .bundle directory in the workspace directory. This means, however, that potential gem updates could break the build unnoticably and that when one wipes the workspace an initial gem install needs to be executed. In the latter case we need to update the check tasks to make sure that 'bundle install' had been run.