Update jQuery to v3.4.0

[coliff]

minimal-mistakes usually uses the latest available version of jQuery 3.x.

jQuery 3.4.0 is now available - I suggest it is updated as this has a minor vulnerability fix and a few performance improvements.

REF:

• https://blog.jquery.com/2019/04/10/jquery-3-4-0-released/
• https://snyk.io/vuln/npm:jquery