chore: bump parent version to 15.6.0 (#9)

Author: marevol

pom.xml

@@ -15,7 +15,7 @@
 	<parent>
 		<groupId>org.codelibs.fess</groupId>
 		<artifactId>fess-parent</artifactId>
-		<version>15.6.0-SNAPSHOT</version>
+		<version>15.6.0</version>
 		<relativePath />
 	</parent>
 	<build>