go build is failing on master

[Hexcles]

Example failure: https://travis-ci.com/web-platform-tests/wpt.fyi/jobs/190448983

I'm almost 100% sure the most recent commit did not break this. The working theory is that some dependency upgraded recently and started to use unsafe stuff, which is not supported on AppEngine standard. This theory is supported by the fact I can still build the go app locally.

[Hexcles]

I think the offender is a recent change in logrus. Filed sirupsen/logrus#947

[Hexcles]

If the maintainer of logrus doesn't fix it this week, I'll try pinning the version of logrus in this project.

[Hexcles]

Posted a fix to upstream: sirupsen/logrus#948

Failing that, I might fork logrus until we migrate to AppEngine Go 1.11 (there's no easy way to vendor external dependencies or use go mod on AppEngine Go 1.9).