FOP-2856: Support compile on Java 12

git-svn-id: https://svn.apache.org/repos/asf/xmlgraphics/fop-pdf-images/trunk@1856923 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/build.properties b/build.properties
index b0fafc9..64d18f0 100644
--- a/build.properties
+++ b/build.properties
@@ -5,7 +5,7 @@
 javac.debug=on
 javac.deprecation=on
 javac.fork=off
-javac.source=1.6
-javac.target=1.6
+javac.source=1.7
+javac.target=1.7
 
 junit.fork=on
\ No newline at end of file