Allow java 9 and 10

[victorwss]

I'm trying to use Checker framework in a Java 10 project, but the plugin doesn't allow versions other than 7 and 8:

gradle-checker-framework-plugin/src/main/groovy/com/jaredsburrows/checkerframework/CheckerPlugin.groovy

Lines 37 to 46 in a7cc979

	private static configureProject(def project) {
	// Check for Java 7 or Java 8 to make sure to get correct annotations dependency
	def jdkVersion
	if (JavaVersion.current().java7) {
	jdkVersion = ANNOTATED_JDK_NAME_JDK7
	} else if (JavaVersion.current().java8) {
	jdkVersion = ANNOTATED_JDK_NAME_JDK8
	} else {
	throw new IllegalStateException("Checker plugin only supports Java 7 and Java 8 projects.")
	}

Why can't it support Java >= 9?

[grv87]

Checker Framework doesn't support Java 9 and 10 yet.
See typetools/checker-framework#1224

[jaredsburrows]

Thanks @grv87.

@victorwss Im closing for now. Feel free to re-open when checkerframework supports Java 9+.