Same as here.
Can you try this?
<properties>
<gae.version>1.7.5</gae.version>
<gae-runtime.version>1.7.5.1</gae-runtime.version>
</properties>
<plugin>
<groupId>net.kindleit</groupId>
<artifactId>maven-gae-plugin</artifactId>
<version>${maven.gae.plugin.version}</version>
<configuration>
<unpackVersion>${gae.version}</unpackVersion>
<serverId>appengine.google.com</serverId>
<appDir>${webappDirectory}</appDir>
</configuration>
<dependencies>
<dependency>
<groupId>net.kindleit</groupId>
<artifactId>gae-runtime</artifactId>
<version>${gae-runtime.version}</version>
<type>pom</type>
</dependency>
</dependencies>
</plugin>