Updated my old Rails project to Rails 3.2.10 (deployed on Heroku using WebSolr) then I suddenly got some rake issues starting Solr locally (development mode). Went around Google for some time and ended up with this setup:
gem 'sunspotrails', '~> 1.3.0'
group :development do
gem 'sunspotsolr', :git => 'git://github.com/sunspot/sunspot.git'
And added the line below on the Rakefile