Update dependencies causing audit warnings on install

[DavidVujic]

Description

Updated dependencies and code according to new lint rules.

Motivation and Context

npm install caused audit warnings: found 15 vulnerabilities (11 low, 4 high)

How Has This Been Tested?

npm run lint passed
npm test passed
Ran the examples according to https://github.com/yfinkelstein/node-zookeeper/tree/master/examples#examples
Ran the integration tests according to https://github.com/yfinkelstein/node-zookeeper/tree/master/tests/integration
Travis-CI build passed

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I have read the code of conduct.
• I have read the contributing guide.
• I have updated the documentation accordingly (if applicable).