Check playground examples before deploying

[mboes]

Is your feature request related to a problem? Please describe.
We show example Nickel code on the front page. Before deploying, we need to check that these examples work as intended using the latest stable release of Nickel (or latest master until we have such a release). That's work.

Describe the solution you'd like
We should automate checking that the example code works as intended as part of pre-deploy automatic checks.