Tell Rails which version of gems to use

How do you tell Rails what version of gems to use?

For example, I have active record 1.14.2, 1.14.4, 1.15.1, and 1.15.3
installed. 1.15.1 is actually what is used when I browse the website.
I want it to use 1.15.3...

The same goes for active support as well.

In config/environment.rb...

# Specifies gem version of Rails to use when vendor/rails is not present
RAILS_GEM_VERSION = '1.1.6'

Pick the rails version that corresponds to the "sub versions" you want to use...

-philip