BUG - Pinned-Dependencies should not look into vendor directories

**Describe the bug**
go run . --repo=github.com/hyperledger/fabric --checks=Pinned-Dependencies --show-details


```                          
vendor/github.com/nxadm/tail/Dockerfile:
```

```
vendor/github.com/nxadm/tail/Dockerfile:                                                                                        
'go get -v github.com/nxadm/tail'
```