Update skipper build for Go 1.13

[mikkeloscar]

Updates skipper build for go 1.13

I basically ran go mod tidy with Go 1.13 to ensure that there are no changes to go.mod during build which would make the build "dirty" in git. :)

[mikkeloscar]

Added an update which moves the flag parsing to be called in func main rather than in func init. Without this change the following fails:

go test -race -test.short -run ^Test github.com/zalando/skipper/cmd/skipper
flag provided but not defined: -test.testlogfile
Usage of /tmp/go-build634583681/b001/skipper.test:

I'm not completely sure on the impact of this change but considering it's bad practice to do a lot of things in init causing problems like this, it probably makes sense :)

The test.testlogfile flag comes from: https://github.com/golang/go/blob/befa17c5f4591e180e5034fd4e8ca508f52dc7bc/src/cmd/go/internal/test/test.go#L1122

[szuecs]

Disabled staticcheck for now, upstream issue dominikh/go-tools#575

[szuecs]

Go 1.13 has TLS1.3 #745

[szuecs]

👍

[mikkeloscar]

👍