Move mocha to devDependencies

[sergeychernyshev]

Mocha is only used in development (e.g. testing), but is in dependencies in package.json

Snyk reports security issues in dependencies for mocha which in turn cause projects that depend on webpagetest npm module to also be marked as vulnerable, see https://snyk.io/test/npm/webpagetest/0.3.8

[sergeychernyshev]

Didn't realize mocha is used in the project itself, closing this ticket. Might still need to use newer version ;)