update document

pom.xml

 		<groovy.version>2.5.6</groovy.version>
 		<quartz.version>2.3.1</quartz.version>
 
+		<maven-source-plugin.version>3.0.1</maven-source-plugin.version>
+		<maven-javadoc-plugin.version>3.1.0</maven-javadoc-plugin.version>
+		<maven-gpg-plugin.version>1.6</maven-gpg-plugin.version>
+		<maven-war-plugin.version>3.2.2</maven-war-plugin.version>
 	</properties>
 
 	<build>
 					<plugin>
 						<groupId>org.apache.maven.plugins</groupId>
 						<artifactId>maven-source-plugin</artifactId>
-						<version>3.0.1</version>
+						<version>${maven-source-plugin.version}</version>
 						<executions>
 							<execution>
 								<phase>package</phase>
 					<plugin>
 						<groupId>org.apache.maven.plugins</groupId>
 						<artifactId>maven-javadoc-plugin</artifactId>
-						<version>3.0.0</version>
+						<version>${maven-javadoc-plugin.version}</version>
 						<executions>
 							<execution>
 								<phase>package</phase>
                     <plugin>
                         <groupId>org.apache.maven.plugins</groupId>
                         <artifactId>maven-gpg-plugin</artifactId>
-                        <version>1.6</version>
+                        <version>${maven-gpg-plugin.version}</version>
 						<configuration>
 							<useAgent>false</useAgent>
 						</configuration>

xxl-job-executor-samples/xxl-job-executor-sample-spring/pom.xml

 			<plugin>
 				<groupId>org.apache.maven.plugins</groupId>
 				<artifactId>maven-war-plugin</artifactId>
-				<version>2.6</version>
+				<version>${maven-war-plugin.version}</version>
 				<configuration>
 					<archiveClasses>false</archiveClasses>
 				</configuration>