Incompatibility with cfssl/csr

Presumably since b8be2da153c6e2661275ae41b86c8b54ef0e02c6, projects using both cfssl and certmgr seems to fail to build.

To reproduce:

1. Create main.go with:
```go
package main

import (
"fmt"

"github.com/cloudflare/certmgr/cert"
"github.com/cloudflare/cfssl/csr"
)

func main() {
c := csr.New()
x := &cert.Spec{
Request: c,
Key: &cert.File{},
Cert: &cert.File{},
}
fmt.Println(x)
}
```
2. Run (1.12 -> 1.11 has the same issue):
```
docker run --rm -it -v $PWD:/go/src/mytest -w /go/src/mytest golang:1.12 go get -v
```
3. Observe:
```
./main.go:13:3: cannot use c (type *"github.com/cloudflare/cfssl/csr".CertificateRequest) as type *"github.com/cloudflare/certmgr/vendor/github.com/cloudflare/cfssl/csr".CertificateRequest in field value
./main.go:15:3: cannot use cert.File literal (type *cert.File) as type *cert.CertificateFile in field value
```