Bump xstream from 1.4.11.1 to 1.4.15

diff --git a/pom.xml b/pom.xml
index 574e723..5f46260 100644
--- a/pom.xml
+++ b/pom.xml
@@ -193,7 +193,7 @@
     <dependency>
       <groupId>com.thoughtworks.xstream</groupId>
       <artifactId>xstream</artifactId>
-      <version>1.4.11.1</version>
+      <version>1.4.15</version>
     </dependency>
     <dependency>
       <groupId>org.apache.commons</groupId>