fix image comparison

Version 6.8.8-9 of ImageMagick's compare returns 2 on error, 0 if the images are similar, and 1 if they are dissimilar. Rather than using the output of compare, this changes the code to use the actual return code.

When using compare, the actual output of the command is "0 @ 0,0" if the images are similar. I'm presuming, based on the previous code, that "0" was output to STDOUT (or STDERR) if the images were similar.

Unfortunately I don't really understand the purpose that `-subimage-search` was serving in the code, but on removal, these tests go from:
```
Zucchini::Screenshot
  general
    compare
      images are different
        should have a failed indicator in the diff (FAILED - 1)
        should have a failed indicator in the diff with no screen mask (FAILED - 2)
```
to passing on my system.