There is a gem for that!

Thanks to Patrick Lenz who wrote bundle_outdated (gem install bundle_outdated).

In your project directory, execute bundle-outdated:

$ bundle-outdated 
Finding outdated gems..
 Newer versions found for:
 sass-rails (3.2.5 > 3.2.3)
 coffee-rails (3.2.2 > 3.2.1)
 uglifier (1.2.4 > 1.0.3)

Lock bundle to these versions by putting the following in your Gemfile:
 gem 'sass-rails', '3.2.5'
 gem 'coffee-rails', '3.2.2'
 gem 'uglifier', '1.2.4'

You may try to update non-specific dependencies via:
 $ bundle update sass-rails coffee-rails uglifier

Handwaving specifications:
 sass-rails: ~> 3.2.3
 coffee-rails: ~> 3.2.1
 uglifier: >= 1.0.3