Update CHROME_BIN to detect chromium on Debian Jessie
This override stopped working after the transition to Nodepool
instances running Jessie because Chromium is installed there
as 'chromium' instead of 'chromium-browser'.
However, this didn't matter because we've long upgrade to
karma-chrome-launcher v0.1.8 and later, which detects both Ubuntu
and Debian Chromium distributions by default.
However, this is changing in karma-chrome-launcher v2.0.0.
See I6ba0d971e7e8 in mediawiki/core.git.