restored Java 9 build (#1586)

pom.xml

@@ -64,11 +64,32 @@
           <groupId>org.apache.maven.plugins</groupId>
           <artifactId>maven-compiler-plugin</artifactId>
           <version>3.8.1</version>
+          <executions>
+            <execution>
+              <id>default-compile</id>
+              <configuration>
+                <jdkToolchain>
+                  <version>9</version>
+                </jdkToolchain>
+                <release>9</release>
+              </configuration>
+            </execution>
+            <execution>
+              <id>base-compile</id>
+              <goals>
+                <goal>compile</goal>
+              </goals>
+              <configuration>
+                <excludes>
+                  <exclude>module-info.java</exclude>
+                </excludes>
+              </configuration>
+            </execution>
+          </executions>
           <configuration>
-            <excludes>
-              <release>6</release>
-              <exclude>**/module-info.java</exclude>
-            </excludes>
+            <jdkToolchain>
+              <version>[1.5,9)</version>
+            </jdkToolchain>
             <source>1.6</source>
             <target>1.6</target>
           </configuration>