Upgrade ivy.api.reference version to 2.5.0

Contributed-by: Gintautas Grigelionis
Pull request: https://github.com/apache/ant-ivy/pull/90/
diff --git a/build.properties b/build.properties
index 72d4600..cbc3226 100644
--- a/build.properties
+++ b/build.properties
@@ -47,7 +47,7 @@
 ivy.minimum.javaversion=1.8
 debug.mode=on
 ivy.install.version=1.4.1
-ivy.api.reference=2.4.0
+ivy.api.reference=2.5.0
 
 #status=integration