chore(deps): Upgrade to mocha@5

[spencerwilson-optimizely]

Summary

• Upgrade mocha 2.5.3 -> 5.2.0

Repro this branch with

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)

For example, expand the npm install step of this PR's node 10 build: link.

Test plan

CI

[coveralls]

Coverage remained the same at 97.11% when pulling c4fe79f on sw/upgrade-mocha into 49e1e08 on master.