Update just one gem with bundler

You simply need to specify the gem name on the command line:

bundle update gem-name

Here you can find a good explanation on the difference between

Update both gem and dependencies:

bundle update gem-name 

or

Update exclusively the gem:

bundle update --source gem-name

along with some nice examples of possible side-effects.

Update

As @Tim's answer says, as of Bundler 1.14 the officially-supported way to this is with bundle update --conservative gem-name.

The way to do this is to run the following command:

bundle update --source gem-name

It appears that with newer versions of bundler (>= 1.14) it's:

bundle update --conservative gem-name