(RE-2116) Build gem in a tmpdir

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 commit updates the create_gem
method to first make a tmp directory, then copy all of the gem files
into it, and build the gem.
(RE-2116) Burn in version during gem creation

Previously, burning in the version during gem creation was not possible.
With the previous commit to build gems in a tmp dir, this is now
possible. This commit adds an invocation of the versionbump method if
it is required.