Fix a bug of pip.list

Before:
```
$ salt-call pip.list tinypng
salt-dev:
    git+https://github.com/saltstack/salt.git@77e758c2e8142d92298aa26db08ed6af02506038
tinypng:
    1.2.0
```

After:
```
$ salt-call pip.list tinypng
tinypng:
    1.2.0
```