travis: prebuild with -cover only.
travis: prebuild with -cover only.

Using go test -i -race ./... slowed the build down. I think go test -i ./...
doesn't help with using -coverprofile so trying with the -cover flag this time.

Timing:
- for loop with go test -coverprofile: 10~11s
- go test -race ./...: 5s
- go test -i ./...: 0.13s so it's useless
- go test -i -race ./...: 3.6s
- go test -race ./... after prebuild: 3s

Prebuilding for -race makes it significantly slower overall.

google/periph