exec java gives -1 exit code on windows, running java -version is bet…

…ter check

lib/veewee/provider/virtualbox/box/build.rb

@@ -7,7 +7,7 @@ def build(options={})
           download_vbox_guest_additions_iso(options)
 
           unless definition.floppy_files.nil?
-            unless self.shell_exec("java").status == 0
+            unless self.shell_exec("java -version").status == 0
               raise Veewee::Error,"This box contains floppyfiles, to create it you require to have java installed or have it in your path"
             end
           end