The vendored jquery version was 1.9.1 from 2013-02-04. Let's replace it with the
most recent one from the 1.x branch (1.12.4 from 2016-05-20).
The modification in rjquery.js is needed because recent jQuery versions changed
their behaviour, and do not set themselves on the global window object.
See: https://github.com/parcel-bundler/parcel/issues/333#issuecomment-357882648
This will be the lastest jQuery 1.x version ever, because 1.x branch is
definitively EOLed (see https://github.com/jquery/jquery.com/issues/162).
This is a stopgap measure to get the latest security fixes. Going forward,
another strategy will be needed.
Closes#3640
