Add 3,15 to DOAP file

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1761823 13f79535-47bb-0310-9956-ffa450edef68

doap_POI.rdf

@@ -35,6 +35,13 @@
     <programming-language>Java</programming-language>
     <category rdf:resource="http://projects.apache.org/category/content" />
     <category rdf:resource="http://projects.apache.org/category/library" />
+    <release>
+      <Version>
+        <name>Apache POI 3.15</name>
+        <created>2016-09-21</created>
+        <revision>3.15</revision>
+      </Version>
+    </release>
     <release>
       <Version>
         <name>Apache POI 3.14</name>