(RE-2116) Build gem in a tmpdir and burn in version

Previously, the gem was built out of the project_root, which worked well
as long as you didn't want to manipulate any of the files in the gem,
like to burn in a version string. This PR updates the create_gem
method to first make a tmp directory, then copy all of the gem files
into it, and build the gem.

Previously, burning in the version during gem creation was not possible.
This PR also adds an invocation of the versionbump method if
it is required.