updates mercury to the latest version

For some reason it was breaking on my computer on 8.0.0, uprading to
9.1.0 seems to fix the issue for me. The problem was with the
`patchObject` method, using `previousValue` but it was undefined.

Oddly enough @dominictarr had the same version and it was working for
him though.
Merge pull request #172 from rcsole/update-mercury

updates mercury to the latest version