Remove Babel

[mearns]

I'm tired of maintaining babel in this project. It's on the old babel v6 and the massive dependency tree ends aways ends up with security issues. And it's really not needed for such a small project.

Acceptance Criteria

This ticket is complete when the following statements are true of this codebase:

• There are no babel related dependencies in package.json.
• There are no npm scripts defined that use babel.
• All "compile*" scripts have been removed from package.json
• The "stryker.conf.js" file no longer uses babel.
• The travis-ci file no longer contains matrix entries for NODE_ENVIRONMENT=development
• The travis-ci matrix no longer sets the NODE_ENVIRONMENT.
• The travis-ci file uses the build script instead of compile test (any entries that use just compile can be removed).

Definition of Done

Additionally, the following should all be true as a matter of course:

• All existing unit tests pass on all supported Node versions (>=6 <13).
• Mutation tests pass (npm run mutation-test finds "No surviving mutants at all.").
• No npm audit issues exist.
• package-lock.json is up to date.

You may, optionally, expand the supported node versions up through the current LTS version. If you do so, add it to the travis CI matrix, add it to the engines property in package.json, and make sure it actually works.