fix Rakefile on RHEL 6
fix Rakefile on RHEL 6

On RHEL 6's ruby193 SCL, the bundler/gem_tasks requirement throws a
LoadError because it fails to find thor. Skip over this exact error so
that eg. "bundle exec rake test" will not bail out on that platform.

rae/gitorious-libdolt,rae/gitorious-libdolt,Gitorious-backup/libdolt,Gitorious-backup/libdolt