Use https for JDK javadoc links.

Conditionally adds a `--release=7` flag to tell javadoc the source compatibility. This inhibits javadoc in later JDKs erroring out expecting modules when there are none.

Fixes #7.

Author: Capstan

project.gradle

@@ -33,7 +33,15 @@ dependencies {
         version: "2.0.1");
 }
 
-// FIXME: For some reason, making this https causes OpenJDK 11 to fail.
-javadoc.options.links("http://docs.oracle.com/javase/7/docs/api/");
-javadoc.options.links("https://www.javadoc.io/doc/com.google.code.findbugs/jsr305/3.0.1/");
+javadoc {
+    options {
+        def currentJavaVersion = org.gradle.api.JavaVersion.current()
+        // FIXME: https://github.com/gradle/gradle/issues/11182
+        if (currentJavaVersion.compareTo(org.gradle.api.JavaVersion.VERSION_1_9) >= 0) {
+            addStringOption("-release", "7");
+        }
+        links("https://docs.oracle.com/javase/7/docs/api/");
+        links("https://www.javadoc.io/doc/com.google.code.findbugs/jsr305/3.0.1/");
+    }
+}