`pod` shows help when there's no Podfile found

This should be a non-breaking fix for #1771.

```
$ pod
CocoaPods, the Objective-C library package manager.

Commands:

    * help       Show help for the given command.
<SKIP>
$ touch Podfile
$ pod
Analyzing dependencies
<SKIP>
```

cc/ #1952 & @dbgrandi