chore(deps): update devDependency eslint to 6.8

[QmarkC]

This PR downgrades devDependency eslint from v7.0 to v6.8.0 and resolves the npm install warnings. In #272 I had matched the semver used for yargs dependencies but yargs had prematurely upgraded eslint.

npm WARN @typescript-eslint/eslint-plugin@2.34.0 requires a peer of eslint@^5.0.0 || ^6.0.0 but none is installed. You must install peer dependencies yourself.
npm WARN @typescript-eslint/parser@2.34.0 requires a peer of eslint@^5.0.0 || ^6.0.0 but none is installed. You must install peer dependencies yourself.
npm WARN eslint-plugin-react@7.14.3 requires a peer of eslint@^3.0.0 || ^4.0.0 || ^5.0.0 || ^6.0.0 but none is installed. You must install peer dependencies yourself.