update version of go tools to fix build?

Created by: nicksnyder

fixes broken builds like

https://buildkite.com/sourcegraph/sourcegraph/builds/23082#ade59a27-cd2d-4643-8e4c-21318fe0d81a

where we see the error

go: finding honnef.co/go/tools v0.0.0-20180910201051-f1b53a58b022
go: honnef.co/go/[email protected]: unexpected status (http://athens-athens-proxy/honnef.co/go/tools/@v/v0.0.0-20180910201051-f1b53a58b022.info): 500 Internal Server Error

All I did was run go get -u honnef.co/go/tools