Only use SonarQube with Java 8

It fails on Java 7, and is unsupported on Java 9.

.travis.yml

 language: java
 
 jdk:
-  - oraclejdk8
+  - oraclejdk9
   - openjdk8
 
 matrix:
   include:
-    - jdk: oraclejdk9
+    - jdk: oraclejdk8
       addons:
         sonarcloud:
           organization: "i2p"
@@ -29,7 +29,7 @@ cache:
 
 script:
   - |
-    if [ "$TRAVIS_JDK_VERSION" == "oraclejdk9" ]; then
+    if [ "$TRAVIS_JDK_VERSION" == "oraclejdk8" ]; then
       ./gradlew sonarqube codeCoverageReport
     else
       ./gradlew check codeCoverageReport

build.gradle

-plugins {
-    id "org.sonarqube" version "2.6.1"
+// SonarQube is currently only supported on Java 8
+// https://docs.sonarqube.org/display/SONAR/Requirements#Requirements-SupportedPlatforms
+if (JavaVersion.current() == JavaVersion.VERSION_1_8) {
+    plugins {
+        id "org.sonarqube" version "2.6.1"
+    }
 }
 
 // Exclude apps/ dir itself, but include its subdirs