Isabelle/PIDE: potential problem with Ubuntu Linux 18.04

The kernel update linux-image-4.15.0-36-generic (Oct-2018) introduces a timing problem with socket communication in the Isabelle Prover IDE, notably Isabelle/jEdit. Thus loading big sessions becomes very slow (e.g. theory HOL-Library.Library or HOL-Analysis.Analysis).

This can be avoided by downgrading to linux-image-4.15.0-34-generic or by using the following temporary workaround for Isabelle2018.