Please enable automated monthly build/test on Travis CI via cron jobs

[mbrukman]

Right now, Travis CI runs only for pull requests and pushes to branches; however, in times of inactivity¹, there's a possibility for bitrot when the build doesn't run for a very long time, and it's unclear what change in the environment caused a problem.

Travis CI has a feature called cron jobs:

which can automatically run a build every month on a selected branch; presumably, develop is the right choice for this project since that's where development happens, while the master branch is the latest stable release. To improve efficiency, you can enable the option to only run a cron job build if there hasn't been a build already run recently on that branch (see the drop-down next to "Options").

Thanks!

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¹ Such as the most recent case, where there haven't been commits for a year; see PR #413, issue #414, and issue #415 for context.