Fix docgen with correct path to gem's

Fixes #2251
Add creation of zipfile for the artifacts

Fixes #2221
Make the test plugins install throw bundler, so we don't have to install
them every time we run the test (it makes the thing slow). Refactor also
the bundler install task into a single one, making calls to it when
necessary.
make sure the bundler task is just used once while bootstraping tests