Set up nightly travis jobs to identify problems as soon as possible

[jdm]

Right now the linux travis jobs on #908 are failing because a python package has been marked insecure, and this failure is unrelated to the PR. We should run nightly jobs so we are less confused about failures that appear in PR that are caused by external changes that happened since the last PR.

[jdm]

https://travis-ci.org/servo/saltfs/builds . Too bad it's not set up to notify anybody when it fails.