Tilt (Bundler dependency error)

I need to upgrade my apps to Rails 3.2.16, when I did bundle update rails it gives me the following error.

Bundler could not find compatible versions for gem "tilt":
  In Gemfile:
    sass-rails (= 3.2.6) ruby depends on
      tilt (~> 1.3) ruby

    slim (>= 0) ruby depends on
      tilt (2.0.0)

My Gemfile:

gem 'sass-rails',   '~> 3.2.6'
gem 'slim'

I don't know how to fix this. Help please.

Thanks,