Upgrade to snakeyaml >= 1.26 version for CVE-2017-18640 fix

patch by Rahul Nandi; reviewed by Alex Petrov, David Capwell for CASSANDRA-16150

CHANGES.txt

@@ -18,6 +18,7 @@
  * NPE thrown while updating speculative execution time if keyspace is removed during task execution (CASSANDRA-15949)
  * Show the progress of data streaming and index build (CASSANDRA-15406)
  * Add flag to disable chunk cache and disable by default (CASSANDRA-16036)
+ * Upgrade to snakeyaml >= 1.26 version for CVE-2017-18640 fix (CASSANDRA-16150)
 Merged from 3.11:
  * Fix memory leak in CompressedChunkReader (CASSANDRA-15880)
  * Don't attempt value skipping with mixed version cluster (CASSANDRA-15833)

build.xml

@@ -583,8 +583,7 @@
           <dependency groupId="com.googlecode.json-simple" artifactId="json-simple" version="1.1"/>
           <dependency groupId="com.boundary" artifactId="high-scale-lib" version="1.0.6"/>
           <dependency groupId="com.github.jbellis" artifactId="jamm" version="${jamm.version}"/>
-
-          <dependency groupId="org.yaml" artifactId="snakeyaml" version="1.23"/>
+          <dependency groupId="org.yaml" artifactId="snakeyaml" version="1.26"/>
           <dependency groupId="junit" artifactId="junit" version="4.12" />
           <dependency groupId="org.mockito" artifactId="mockito-core" version="3.2.4" />
           <dependency groupId="org.quicktheories" artifactId="quicktheories" version="0.25" />