Add Java 11 test configs (#239)

* Add Java 11 test configs

* Update README badge

* Update maven-javadoc-plugin for java 11

.kokoro/continuous/java11.cfg

@@ -0,0 +1,7 @@
+# Format: //devtools/kokoro/config/proto/build.proto
+
+# Configure the docker image for kokoro-trampoline.
+env_vars: {
+  key: "TRAMPOLINE_IMAGE"
+  value: "gcr.io/cloud-devrel-kokoro-resources/java11"
+}

.kokoro/presubmit/java11.cfg

@@ -0,0 +1,7 @@
+# Format: //devtools/kokoro/config/proto/build.proto
+
+# Configure the docker image for kokoro-trampoline.
+env_vars: {
+  key: "TRAMPOLINE_IMAGE"
+  value: "gcr.io/cloud-devrel-kokoro-resources/java11"
+}

README.md

@@ -42,7 +42,7 @@ Java Version | Status
 ------------ | ------
 Java 7 | [![Kokoro CI](http://storage.googleapis.com/cloud-devrel-public/java/badges/google-oauth-java-client/java7.svg)](https://storage.googleapis.com/cloud-devrel-public/java/badges/google-oauth-java-client/java7.html)
 Java 8 | [![Kokoro CI](http://storage.googleapis.com/cloud-devrel-public/java/badges/google-oauth-java-client/java8.svg)](https://storage.googleapis.com/cloud-devrel-public/java/badges/google-oauth-java-client/java8.html)
-Java 10 | [![Kokoro CI](http://storage.googleapis.com/cloud-devrel-public/java/badges/google-oauth-java-client/java10.svg)](https://storage.googleapis.com/cloud-devrel-public/java/badges/google-oauth-java-client/java10.html)
+Java 11 | [![Kokoro CI](http://storage.googleapis.com/cloud-devrel-public/java/badges/google-oauth-java-client/java11.svg)](https://storage.googleapis.com/cloud-devrel-public/java/badges/google-oauth-java-client/java11.html)
 
 ## Links
 

pom.xml

@@ -240,7 +240,7 @@
         <plugin>
           <groupId>org.apache.maven.plugins</groupId>
           <artifactId>maven-javadoc-plugin</artifactId>
-          <version>2.9.1</version>
+          <version>3.0.1</version>
           <executions>
             <execution>
               <id>attach-javadocs</id>
@@ -300,7 +300,7 @@
       <plugin>
         <artifactId>maven-javadoc-plugin</artifactId>
         <configuration>
-          <additionalparam>-Xdoclint:none</additionalparam>
+          <doclint>none</doclint>
         </configuration>
         <executions>
           <execution>