Update all packages to the most recent version

To support this, the following changes have to be made as well:

- Ignore `package-lock.json` since NPM creates it, but we should not
  have it in the repository.
- Switch from `babel-preset-es2015` to `babel-preset-env` to resolve
  the deprecation warning in the test logs. The latter is more recent
  and flexible, but should be the same functionality-wise.
- `transform` now needs to have the `utf-8` encoding option provided.
  If not given, it will call the callback with a `Buffer` object,
  which results in an unhandled promise rejection since what is
  returned from the callback is a string, not a `Buffer`.

.gitignore

@@ -7,3 +7,4 @@ Makefile
 node_modules/
 examples/node/svgdump/
 examples/node/pdf2png/*.png
+package-lock.json