Add version check for JDK9 and beyond.

Motivation:

Netty's platform dependent parts should know about JDK9.

Modifications:

JDK9 introduce Runtime$Version Runtime.version() which has an int major()
method that always return the major Java version. I call that method to
get the Java major version.

Result:

Netty will recognize all future JDK versions.

common/src/main/java/io/netty/util/internal/PlatformDependent.java

@@ -1030,6 +1030,15 @@ public final class PlatformDependent {
                 break;
             }
 
+            try {
+                Method getVersion = java.lang.Runtime.class.getMethod("version");
+                Object version = getVersion.invoke(null);
+                javaVersion = (Integer) version.getClass().getMethod("major").invoke(version);
+                break;
+            } catch (Throwable ignored) {
+                // Ignore
+            }
+
             try {
                 Class.forName("java.time.Clock", false, getClassLoader(Object.class));
                 javaVersion = 8;