Rails 3: How can one update Rspec 2.3.0 to 2.4.0 ?

I've tried putting it on my Gemfile and got:

Fetching source index for http://rubygems.org/ You have requested: rspec = 2.4.0

The bun开发者_运维知识库dle currently has rspec locked at 2.3.0.

Try running bundle update rspec in your project directory, and then read the bundler website on updating a dependency.