fix: set GOSUMDB off

[kuisathaverat]

Disable a new feature on go 1.13 that it is failing in the CI

closses #630

[kuisathaverat]

The behavior changes after add GOPROXY=direct and GOSUMDB=off, now seems a dependencies issue

[2019-09-04T15:01:27.464Z] go: finding gopkg.in/src-d/go-git-fixtures.v3 v3.3.0
[2019-09-04T15:01:35.596Z] *** old
[2019-09-04T15:01:35.596Z] --- new
[2019-09-04T15:01:35.596Z] ***************
[2019-09-04T15:01:35.596Z] *** 3,5 ****
[2019-09-04T15:01:35.596Z] --- 3,7 ----
[2019-09-04T15:01:35.596Z]   require go.elastic.co/apm v1.5.0
[2019-09-04T15:01:35.596Z]   
[2019-09-04T15:01:35.596Z]   replace go.elastic.co/apm => /var/lib/jenkins/workspace/agent-go_apm-agent-go-mbp_PR-631/src/go.elastic.co/apm
[2019-09-04T15:01:35.596Z] + 
[2019-09-04T15:01:35.596Z] + go 1.13
[2019-09-04T15:01:35.596Z] 2019/09/04 15:01:34 exit status 1
[2019-09-04T15:01:35.596Z] exit status 1
[2019-09-04T15:01:35.596Z] Makefile:29: recipe for target 'check-modules' failed
[2019-09-04T15:01:35.596Z] make: *** [check-modules] Error 1

@axw I'd need your help here

[axw]

I've fixed the issue in #629. The "genmod" program intentionally sets the proxy to an invalid hostname, to cause any attempt to download modules to fail - the idea being to ensure no dependencies are missing from our go.mod files. Go 1.13 changed some behaviour so that the "GOSUMDB" default was derived from GOPROXY.