Update ffmpeg options

Newer versions of ffmpeg (post >= 1.1) require `-cmp` argument to not contain a leading plus sign.

Here is the error you get if + is present:

```bash
[NULL @ 0x25d2760] [Eval @ 0x7fff532a6700] Undefined constant or missing '(' in 'chroma'
[NULL @ 0x25d2760] Unable to parse option value "+chroma"
[NULL @ 0x25d2760] Error setting option cmp to value +chroma.
```