Updated multi_json Ruby gem

This shoud fix "Could not find multi_json-1.7.5 in any of the sources"
error.

Example:
https://wmf.ci.cloudbees.com/job/browsertests-commons.wikimedia.beta.wmf
labs.org-linux-chrome/95/console

Change-Id: Ib85facac79cbc837b1634a4a46e1f6fa2be88594