How to reduce heroku slug size?

My slug size is 89.5MB which is huge.

However the repository size is quite small:

$ du -hsc 
8.0M    . 
8.0M    total

Following this blog post: http://dazedthots.blogspot.com/2011/07/reducing-slug-size-heroku.html , there is a bug on Heroku with changing Gemfiles. Mine has changed several times including git dependencies but now, the only git dependency is "rails_admin". See below:

source 'http://rubygems.org'

gem 'rails',      '~> 3.1.0'
gem 'rails-i18n', '~> 0.1.3'
gem 'pg',         '~> 0.11.0'
gem 'rake',       '~> 0.8.7'
gem 'thin', '~> 1.2.11'
gem 'heroku', '~> 2.4.0'
gem 'squeel',     '~> 0.8.6'
gem 'devise', '~> 1.4.2'
gem 'slim-rails',  '~> 0.2.0'
gem 'simple_form', '~> 1.4.2'
gem 'will_paginate', '~> 3.0'
gem 'sunspot_rails', '~> 1.2.1'
gem 'jquery-rails',    '~> 1.0.12'
gem 'modernizr-rails', '~> 2.0.6'
gem 'rails_admin', git: 'https://github.com/sferik/rails_admin.git'
gem 'sass-rails',   '~> 3.1.0'
gem 'coffee-rails', '~> 3.1.0'
gem 'uglifier',     '~> 1.0.0'
gem 'newrelic_rpm'

I've contacted the Heroku support but no answer until now? Any idea?