All linting passes if flake8 is not installed.

Reproduction steps:

```
$> git clone git@github.com:btholt/grunt-flake8.git
$> cd grunt-flake8
$> sudo pip uninstall flake8
$> grunt flake8
```

All tests will pass, even if there are errors.

It's definitely outside of the scope of the task to try and get flake8 installed for you, but it needs to at least warn you/quit when it runs the command `which flake8`.

It should return the exit code from that `which` call, and display a failure message if that code is equal to 1.