Enable daily Travis CI builds

[JustasB]

Background Whenever a Travis build fails, it can be difficult to determine if the failure is due to the code that was just checked in or due to a change in some external dependency (e.g. an update to a pip package or unavailability of some resource). Scheduled builds could help determine the cause.

Proposal I propose we enable daily scheduled Travis CI builds. This can be done via Travis Cron job settings.

Expected result The scheduled builds would run every 24 hours and if it fails one can inspect Travis CI build history and see one of the following:

• Previous checked-in build succeeded, but the scheduled build failed: Indicates an issue with an external dependency
• Previous checked-in build failed and so did the scheduled build: The failure is probably caused by the earlier checked-in code

[rgerkin]

Good idea. I've just added weekly ones to master and dev branches of both sciunit and neuronunit. I don't want to do daily at the moment since some of the external resources (like neuroelectro) time out periodically and I don't want to get an email every time this happens.