Use source=7, target=1.7 for target Java version

Without this change the build otherwise emits the following warnings:

```
[javac] warning: [options] source value 6 is obsolete and will be removed in a future release
[javac] warning: [options] target value 1.6 is obsolete and will be removed in a future release
```

#243 (comment)
also seems to indicate that in Java 12, source=6/target=1.6 will
apparently become an error case rather than a warning case.

Thanks @innovimax

build.xsl

@@ -13,8 +13,8 @@
     <property name="build.dir" value="${{basedir}}/build"/>
     <property name="doc.dir" value="${{basedir}}/doc"/>
     <property name="javacc.dir" value="${{lib.dir}}"/>
-    <property name="ant.build.javac.source" value="6"/>
-    <property name="ant.build.javac.target" value="1.6"/>
+    <property name="ant.build.javac.source" value="7"/>
+    <property name="ant.build.javac.target" value="1.7"/>
     <taskdef name="testng" classname="org.testng.TestNGAntTask">
       <classpath>
 	<pathelement location="${{lib.dir}}/testng.jar"/>