Fix incompatibility with xstream 1.4.18

[kohlschuetter]

This change adds necessary xstream permissions so we can deserialize Nexus XML responses using the nexus-maven-plugin when using a dependency declaration like this:

<plugin>
  <groupId>org.sonatype.plugins</groupId>
  <artifactId>nexus-staging-maven-plugin</artifactId>
  <version>1.6.8</version>
  <dependencies>
    <dependency>
      <groupId>org.sonatype.nexus</groupId>
      <artifactId>nexus-client-core</artifactId>
      <version>2.14.21-02</version>
    </dependency>
    <dependency>
      <groupId>com.thoughtworks.xstream</groupId>
      <artifactId>xstream</artifactId>
      <version>1.4.18</version>
    </dependency>
    <dependency><!-- needed by xstream 1.4.16 -->
      <groupId>xpp3</groupId>
      <artifactId>xpp3</artifactId>
      <version>1.1.4c</version>
    </dependency>
    <dependency>
      <groupId>org.apache.httpcomponents</groupId>
      <artifactId>httpclient</artifactId>
      <version>4.5.13</version>
    </dependency>
    <dependency>
      <groupId>org.apache.httpcomponents</groupId>
      <artifactId>httpcore</artifactId>
      <version>4.4.15</version>
    </dependency>
    <dependency>
      <groupId>ch.qos.logback</groupId>
      <artifactId>logback-classic</artifactId>
      <version>1.2.9</version>
    </dependency>
    <dependency>
      <groupId>ch.qos.logback</groupId>
      <artifactId>logback-core</artifactId>
      <version>1.2.9</version>
    </dependency>
  </dependencies>
</plugin>

Also see x-stream/xstream#263

[zqfan]

thanks!