Your task service is not getting started because your roles.properties file link. Just look for the following code snippet in your build.xml
<java classname="org.jbpm.DemoTaskService" fork="true">
<classpath>
<pathelement path="${install.home}/task-service/target"/>
<path refid="classpath.human.task" />
</classpath>
</java>
And add the line <sysproperty key="jbpm.user.group.mapping" value="file:///<path of your jboss server>/jboss-as-7.1.1.Final/standalone/configuration/roles.properties"/> like this :
<java classname="org.jbpm.DemoTaskService" fork="true">
<sysproperty key="jbpm.user.group.mapping" value="file:///<path of your jboss server>/jboss-as-7.1.1.Final/standalone/configuration/roles.properties"/>
<classpath>
<pathelement path="${install.home}/task-service/target"/>
<path refid="classpath.human.task" />
</classpath>
</java>