remove warning 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-compiler-plugin is missing.

pom.xml

@@ -17,6 +17,8 @@
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-compiler-plugin</artifactId>
+                <!-- 与Servlet版本对应 -->
+                <version>3.1</version>
                 <configuration>
                     <source>1.6</source>
                     <target>1.6</target>