move mocha to a devDependency

[shiftkey]

The only usage of mocha I could find was in here:

fs-admin/package.json

Line 7 in 7c8a1bd

"test": "standard && mocha"

[shiftkey]

I only noticed this because of CVE-2017-16042 and the version of mocha used in the project depending on the problem version of growl. I'm no longer using mocha for tests on the project, so this is a dependency I no longer need.

I'll follow up with a PR to upgrade mocha after this is merged.