What's the OS version on which the tests are running?

Can you please mention the exact OS version of the VM that the tests are running on? In addition, is it modifiable? My guess is not, as I’ve not found any such configuration.
The context: we’d like to verify that the tests are running on the same OS version (or very similar version) as the production VM.

The current environment is documented here https://circleci.com/docs/environment (unless your participating in the Trusty beta)

An old habit of mine is to always place a call to uname -a at the start of the tests - my Makefiles always have the following target so I can track down problems with changes to the environment.

info:
	@python --version
	@pip --version
	@virtualenv --version
	@uname -a
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