Bump target Java version to 1.6

Java 1.5 is EOL, and JDK 9 drops support for targeting it.

https://lists.andrew.cmu.edu/pipermail/openslide-users/2016-September/001297.html
http://openjdk.java.net/jeps/182
http://www.oracle.com/technetwork/java/eol-135779.html

Author: bgilbert

build.xml

@@ -2,8 +2,8 @@
 <project basedir="." default="jar" name="openslide java">
 	<property environment="env"/>
 	<property name="debuglevel" value="source,lines,vars"/>
-	<property name="target" value="1.5"/>
-	<property name="source" value="1.5"/>
+	<property name="target" value="1.6"/>
+	<property name="source" value="1.6"/>
 	<property name="builddir" value="."/>
 	<property name="openslide.binding.version" value="UNKNOWN"/>
 	<property name="openslide.jni.path" value=""/>