PKG-Gen should take the latest stable CKEditor 5 version

[pomek]

📝 Provide a description of the improvement

Travis reports the following error:

Webpack bundling...
[
  {
    moduleIdentifier: 'javascript/esm|/home/travis/build/ckeditor/ckeditor5-test-package/node_modules/@ckeditor/ckeditor5-core/src/index.js',
    moduleName: './node_modules/@ckeditor/ckeditor5-core/src/index.js',
    loc: '8:0-45',
    message: "Module not found: Error: Can't resolve './plugin' in '/home/travis/build/ckeditor/ckeditor5-test-package/node_modules/@ckeditor/ckeditor5-core/src'\n" +
      "Did you mean 'plugin.js'?\n" +
      "BREAKING CHANGE: The request './plugin' failed to resolve only because it was resolved as fully specified\n" +
      `(probably because the origin is strict EcmaScript Module, e. g. a module with javascript mimetype, a '*.mjs' file, or a '*.js' file where the package.json contains '"type": "module"').\n` +
      'The extension in the request is mandatory for it to be fully specified.\n' +
      'Add the extension to the request.',

https://app.travis-ci.com/github/ckeditor/ckeditor5-package-generator/jobs/605292891#L778

It can be fixed by applying this change - ckeditor/ckeditor5-dev@efcf009 - in webpack configs.

However, I think the generator should not use alpha releases when preparing a new package. Hence, I propose fetching the @latest version instead of the latest release.