Fix ESLint import/no-commonjs issues

[jazhen]

Fix all warnings under the category import/no-commonjs.

List of issues:

• wtt_server/ecosystem.config.js
  • 1:1 warning Expected "export" or "export default" import/no-commonjs
• wtt_server/server/utilities/remove-duplicate-trees.js
  • 3:16 warning Expected "import" instead of "require()" import/no-commonjs
• wtt_server/server/utilities/update-ids.js
  • 2:16 warning Expected "import" instead of "require()" import/no-commonjs
  • 3:23 warning Expected "import" instead of "require()" import/no-commonjs

closes #107

[vernfongchao]

looks good!