Maven: Build with API 2.15.6

Change-Id: I104f65358fa8e5f87fbd603c7453116a13284163
diff --git a/pom.xml b/pom.xml
index 4395cce..970f4a2 100644
--- a/pom.xml
+++ b/pom.xml
@@ -25,7 +25,7 @@
     <groupId>com.cisco.gerrit.plugins</groupId>
     <artifactId>slack-integration</artifactId>
     <packaging>jar</packaging>
-    <version>2.15-rc3</version>
+    <version>2.15.6</version>
     <name>Slack Integration Plugin</name>
 
     <properties>