Security updates, modernized codebase

This release includes bcrypt 5.x, which includes important security updates (required Node 12+).

In addition, the following updates have been made:

• Modernized codebase using ES6 goodies
• Updated dependencies
• More tests and and updated testing framework

Thanks to @DominicRoyStang for his work on this 🙌

2.x Deprecated

The 2.x branch has now been deprecated in favor of the 3.x branch. The 3.x contains bcrypt 5.x, which includes important security updates. Unfortunately, bcrypt 5.x is only compatible with Node 12+ so if you can't upgrade to Node 12+, use this release.