Update gitian instructions to reference a newer version of boxcutter (ubuntu 16.04.4 -> 16.04.5)

Summary:
In the previously referenced commit (changed in the diff) the Ubuntu ISO specified was obsolete and no
longer available for download from Ubuntu. This commit changes the documentation to reference a more
recent commit with an available Ubuntu image.

Test Plan: Manually try building the Ubuntu image with packer as per the instructions

Reviewers: deadalnix, schancel, Fabien, #bitcoin_abc

Reviewed By: schancel, Fabien, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1991