create: `invalidator` not set in generated `.meta/config.json`

[sanderploegsma]

I was playing around with configlet create and noticed that the generated .meta/config.json does not contain the invalidator property, even though it's set in the files object of the track's config.json:

Track config:

{
  "files": {
    "solution": [
      "src/main/java/%{pascal_slug}.java"
    ],
    "test": [
      "src/test/java/%{pascal_slug}Test.java"
    ],
    "example": [
      ".meta/src/reference/java/%{pascal_slug}.java"
    ],
    "exemplar": [
      ".meta/src/reference/java/%{pascal_slug}.java"
    ],
    "invalidator": [
      "build.gradle"
    ]
  }
}

Command:

bin/configlet create --concept-exercise foobar

Generated .meta/config.json:

{
  "authors": [],
  "files": {
    "solution": [
      "src/main/java/Foobar.java"
    ],
    "test": [
      "src/test/java/FoobarTest.java"
    ],
    "exemplar": [
      ".meta/src/reference/java/Foobar.java"
    ]
  },
  "blurb": ""
}

Interestingly enough, it does create the build.gradle file itself.