Change javac task to generate 1.7 classes to ensure Streams 4.0 support

Author: Daniel J. Debrunner

com.ibm.streamsx.json/build.xml

@@ -63,6 +63,7 @@
 	<target name="compile" depends="init">
 		<javac srcdir="${src.dir}" destdir="${build.dir}" debug="true"
 			includeantruntime="no"
+			source="1.7" target="1.7"
 			excludes="com/ibm/streamsx/json/**/*$StreamsModel.java">
 			<classpath>
 				<path refid="cp.compile" />