chore(deps): Upgrade to mocha@5 (#141)

## Summary

- Upgrade mocha 2.5.3 -> 5.2.0

Repro this branch with
```sh
npm install --save-dev mocha@latest
```

Among other things, this addresses the "critical" warning about `growl` visible both here in GitHub and in `npm audit`. The latter is notable because
> npm audit automatically runs when you install a package with npm install

(source: [`npm audit` Getting Started article](https://docs.npmjs.com/getting-started/running-a-security-audit))

For example, expand the `npm install` step of this PR's node 10 build: [link](https://travis-ci.org/optimizely/javascript-sdk/jobs/399773189).
## Test plan
CI

Author: spencerwilson-optimizely

packages/optimizely-sdk/package-lock.json

@@ -46,7 +46,7 @@
     "karma-sinon": "^1.0.5",
     "karma-webpack": "^2.0.3",
     "mathjs": "^3.3.0",
-    "mocha": "^2.5.3",
+    "mocha": "^5.2.0",
     "mocha-lcov-reporter": "^1.3.0",
     "nock": "^7.7.2",
     "sinon": "^2.3.1",