Revert "Fix go vet issues flagged by go 1.9" PR

[jodh-intel]

#587 broke the CI and local unit-test builds. From http://cc-jenkins-ci.westus2.cloudapp.azure.com/job/clear-containers-runtime-azure-ubuntu-17-04/lastBuild/console:

INFO: Running 'go test' as current user on packages '.' with flags '-v -race -timeout 5s'
# github.com/clearcontainers/runtime/vendor/github.com/stretchr/testify/assert
vendor/github.com/stretchr/testify/assert/assertion_forward.go:21: undefined: Conditionf
vendor/github.com/stretchr/testify/assert/assertion_forward.go:45: undefined: Containsf
vendor/github.com/stretchr/testify/assert/assertion_forward.go:65: undefined: Emptyf
vendor/github.com/stretchr/testify/assert/assertion_forward.go:100: undefined: EqualErrorf
vendor/github.com/stretchr/testify/assert/assertion_forward.go:120: undefined: EqualValuesf
vendor/github.com/stretchr/testify/assert/assertion_forward.go:133: undefined: Equalf
vendor/github.com/stretchr/testify/assert/assertion_forward.go:157: undefined: Errorf
vendor/github.com/stretchr/testify/assert/assertion_forward.go:175: undefined: Exactlyf
vendor/github.com/stretchr/testify/assert/assertion_forward.go:190: undefined: FailNowf
vendor/github.com/stretchr/testify/assert/assertion_forward.go:195: undefined: Failf
vendor/github.com/stretchr/testify/assert/assertion_forward.go:195: too many errors
FAIL	github.com/clearcontainers/runtime [build failed]

With go1.8 and go1.9 I could build the runtime, but the tests would fail to compile. I don't understand how this passed under the 17.04 Jenkins CI whereas it now fails under the same system.