update ubuntu hash for gitian

Summary: The old hash does not work anymore. 1e1f4804315b27eba398de93b8024230d190443a is the current master.

Test Plan: follow initial steps of gitian-building.md and create a new virtual box

Reviewers: #bitcoin_abc, deadalnix, schancel

Reviewed By: #bitcoin_abc, deadalnix, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1430