Ensure path is a file before trying to lint.

This task was failing for me with the error:

```
Warning: Unable to read "app/scripts/components/spin.js" file (Error code: EISDIR). Use --force to continue.
```

It appears that app/scripts/components/spin.js was returned from the globbing pattern because it contained ".js". Unfortunately, that's a directory.

I imagine spin.js isn't the only project with a directory name like this, so it seems sensible to check first that the path is a file before trying to lint.