One of the advantages of using Launchpad’s PPA system is that it allows you to easily use the work of others backporting packages to older releases.
For example, the package jgr has a build requirement of libjq. jq is a command-line JSON processor, and the jqr package is required by a small number of other R packages, either directly or indirectly. The Ubuntu packages libjq-dev is available in Bionic, but not be default in Trusty or Xenial.
Luckily a PPA exists with its only purpose to provide backports for this library. This PPA has been added to PPA dependencies for the rebuild of c2d4u for R 3.5. You may need to enable that PPA in your system if you are interested in using packages requiring jq. Thanks to OpenCPU for the jq backport.