Add a module config, twiddle build to support it.

We now have to build with JDK9 or above, but I believe that the built jar files
continue to be compatible with JDK 1.7 or above. If this turns out not to be
the case, please let me know.

pom.xml

@@ -40,11 +40,11 @@
     <developerConnection>scm:git:git@github.com:samskivert/jmustache.git</developerConnection>
     <url>http://github.com/samskivert/jmustache</url>
   </scm>
-
+<!-- 
   <prerequisites>
     <maven>3.3.0</maven>
   </prerequisites>
-
+ -->
   <properties>
     <source.level>1.7</source.level>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
@@ -98,7 +98,7 @@
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-compiler-plugin</artifactId>
-        <version>3.3</version>
+        <version>3.7.0</version>
         <configuration>
           <source>${source.level}</source>
           <target>${source.level}</target>
@@ -116,6 +116,28 @@
             <exclude>**/super/**</exclude>
           </excludes>
         </configuration>
+        <executions>
+          <execution>
+            <id>default-compile</id>
+            <configuration>
+              <includes>
+                <include>module-info.java</include>
+              </includes>
+              <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>
       </plugin>
 
       <plugin>
@@ -181,6 +203,7 @@
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-jar-plugin</artifactId>
+        <version>3.1.1</version>
         <configuration>
           <archive>
             <manifestEntries>