Ensure jQuery 3.x compatibility

Sooner or later, [WordPress core might upgrade to the jQuery 3.x branch](https://core.trac.wordpress.org/ticket/37110).

Informally, I've also heard of situations where sites are required, as a result of security audits, to override core by loading jQuery 3.x, because jQuery 1.2.4 contains an XSS vulnerability.

It might be good to do some testing to ensure that Fieldmanager works with the latest versions of jQuery.

I'm unsure of how to best do that apart from manual testing and feedback. As a start, maybe it's possible to add a run of the JavaScript tests in Travis that forces the use of jQuery 3.x?