This is kind of a hack, but it's for your development env only, so it's Ok ;)
So we are going to upgrade the Solr instance that comes pre-installed with the sunspot_solr gem. Currently its version is 1.3 and I've successfully upgraded it to the 3.5 version. This is a step-by-step about how to do it:
Stop all (in all your envs) your Solr instances with the rake task bundle exec rake sunspot:solr:stop
Move to where the sunspot_solr gem is installed via cd $(bundle show sunspot_solr)
Rename the solr folder to solr-original via mv solr solr-original
Copy the example folder from the Solr distribution into the gem folder via cp -rf ~/Downloads/solr-4.1.0/example solr
That's it, you have upgraded your Solr instance. You can start it: bundle exec rake sunspot:solr:start and reindex bundle exec rake sunspot:solr:reindex. Go to the Solr Admin Dashboard and verify the version number.