FIX ENH: Upgraded pom.xml to compile for Java 11.

pom.xml

@@ -227,10 +227,10 @@
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-compiler-plugin</artifactId>
-        <version>3.6.1</version>
+        <version>3.8.1</version>
         <configuration>
-          <source>1.8</source>
-          <target>1.8</target>
+          <source>11</source>
+          <target>11</target>
         </configuration>
       </plugin>
       <plugin>