Fix CutyCapt path detection

This is a great gem. Thanks for making it.
A small error preventing it from working on my Linux:

The `determine_cutycapt_path` method would do the following:

```ruby
`which CutyCapt`.strip or `which cutycapt`.strip
```

But this would always return the first string regardless of it being empty or not and so would not work if the capitalized `CutyCapt` command wasn't present. This pull request fixes that so it returns the first command found.

I also use your `CutyCaptError` exception if it doesn't find a command.

Thanks,
/Lasse