pip state module requires name even if requirements file is specified

Executing the following state:

```yaml
a set of nameless requirements:
  pip.installed:
    - requirements: salt://requirements.txt
```

produces the following error:

```
----------
    State: - pip
    Name:      a set of nameless requirements
    Function:  installed
        Result:    False
        Comment:   pip raised an exception while parsing 'a set of nameless requirements': ('Expected version spec in', 'a set of nameless requirements', 'at', ' set of nameless requirements')
        Changes:
```

This worked prior to version 0.17.0.

The problem is that the state module tries to install a package called `a set of nameless requirements`. I think a more reasonable behavior would be to ignore the name attribute if `requirements` file is specified. Right now I get around this error by supplying a bogus package to install, like this:

```yaml
a set of nameless requirements:
  pip.installed:
    - name: pip
    - requirements: salt://requirements.txt
```