make sure VISUAL is not empty

if VISUAL is not set, you get weird errors on edit:

```
$ ./stevedore edit python4
./stevedore: 68: ./stevedore: python4/Dockerfile.pre: Permission denied
Uploading context 10240 bytes
Step 1 : FROM stevedore
Step 2 : RUN echo peter:x:1000:1000:Peter van Dijk,,,:/home/peter:/bin/bash >> /etc/passwd
```

This patch makes sure you get vi instead.

On a sidenote: this issue quite clearly shows that stevedore ignores errors. I suggest making stevedore work with sh -e. Happy to open a separate issue for that if you like.