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

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:
```json
{
"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:

```sh
bin/configlet create --concept-exercise foobar
```

Generated `.meta/config.json`:

```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.