WebMagic 启动例子报错

报错内容：　

Exception in thread "main" java.lang.NoClassDefFoundError: org/apache/http/conn/HttpClientConnectionManager
	at us.codecraft.webmagic.downloader.HttpClientDownloader.<init>(HttpClientDownloader.java:38)
	at us.codecraft.webmagic.Spider.initComponent(Spider.java:280)
	at us.codecraft.webmagic.Spider.run(Spider.java:305)
	at us.codecraft.webmagic.model.samples.QQMeishi.main(QQMeishi.java:23)
Caused by: java.lang.ClassNotFoundException: org.apache.http.conn.HttpClientConnectionManager
	at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:338)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
	... 4 more

Process finished with exit code 1

原因：

不详，菜鸟一个，对于Maven还不熟练，在webmagic-parent/pom.xml中明明已经写上了版本号。

解决方法：

在webmagic-core/pom.xml中对httpclient指定版本号，并更新。

<dependency>
    <groupId>org.apache.httpcomponents</groupId>
    <artifactId>httpclient</artifactId>
    <version>4.5.6</version>
</dependency>