Update max supported dependency versions in symbol-check.py

Summary:
This fixes nightly builds, which are currently failing due to the recent addition of threading dependencies.

Need to bump glibc to at least 2.18 to support __cxa_thread_atexit_impl (introduced in D2245)
Bumping to this version should be safe and relatively non-disruptive,
as Jessie was released in 2015 and Trusty was released in 2014.

Test Plan: Run gitian build and verify check-symbols completes successfully in ./var/build.log

Reviewers: Fabien, #bitcoin_abc, deadalnix

Reviewed By: Fabien, #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2331