Only use the Mac OS X JDK-path tweak if the path exists

The new Oracle JVM on Mac OS X doesn't use unconventional paths - only the Apple JVM does. The plugin gives this error when used with the Oracle JVM on Mac OS X:

```
[INFO] java.io.IOException: Can't read [/Library/Java/JavaVirtualMachines/jdk1.7.0_11.jdk/Contents/Home/Classes/classes.jar] (No such file or directory)
[INFO] 	at proguard.InputReader.readInput(InputReader.java:230)
[INFO] 	at proguard.InputReader.readInput(InputReader.java:200)
[INFO] 	at proguard.InputReader.readInput(InputReader.java:178)
[INFO] 	at proguard.InputReader.execute(InputReader.java:100)
[INFO] 	at proguard.ProGuard.readInput(ProGuard.java:196)
[INFO] 	at proguard.ProGuard.execute(ProGuard.java:78)
[INFO] 	at proguard.ProGuard.main(ProGuard.java:492)
[INFO] Caused by: java.io.IOException: No such file or directory
[INFO] 	at proguard.io.DirectoryPump.pumpDataEntries(DirectoryPump.java:50)
[INFO] 	at proguard.InputReader.readInput(InputReader.java:226)
[INFO] 	... 6 more
```

Here's the actual Oracle JVM on Mac OS X:

```
$ ls /Library/Java/JavaVirtualMachines/jdk1.7.0_11.jdk/Contents/Home/Classes/
ls: /Library/Java/JavaVirtualMachines/jdk1.7.0_11.jdk/Contents/Home/Classes/: No such file or directory
$ ls /Library/Java/JavaVirtualMachines/jdk1.7.0_11.jdk/Contents/Home/lib/
ant-javafx.jar		dt.jar			javafx-doclet.jar	jconsole.jar		sa-jdi.jar		visualvm
ct.sym			ir.idl			javafx-mx.jar		orb.idl			tools.jar
$ ls /Library/Java/JavaVirtualMachines/jdk1.7.0_11.jdk/Contents/Home/jre/lib/
JObjC.jar			jli				libjava_crw_demo.dylib		libsaproc.dylib
alt-rt.jar			jsse.jar			libjavafx-font.dylib		libsplashscreen.dylib
applet				jvm.cfg				libjavafx-iio.dylib		libsunec.dylib
calendars.properties		jvm.hprof.txt			libjawt.dylib			libt2k.dylib
charsets.jar			libAppleScriptEngine.dylib	libjdwp.dylib			libunpack.dylib
classlist			libJObjC.dylib			libjfr.dylib			libverify.dylib
cmm				libattach.dylib			libjfxmedia.dylib		libzip.dylib
content-types.properties	libawt.dylib			libjfxwebkit.dylib		logging.properties
currency.data			libdcpr.dylib			libjpeg.dylib			lwawt
deploy.jar			libdecora-sse.dylib		libjsdt.dylib			management
ext				libdeploy.dylib			libjsig.dylib			management-agent.jar
flavormap.properties		libdt_socket.dylib		libjsound.dylib			meta-index
fontconfig.bfc			libfontmanager.dylib		libkcms.dylib			net.properties
fontconfig.properties.src	libglass.dylib			libmanagement.dylib		plugin.jar
fonts				libglib-2.0.0.dylib		libmlib_image.dylib		psfont.properties.ja
fxplugins.dylib			libgstplugins-lite.dylib	libnet.dylib			psfontj2d.properties
headless			libgstreamer-lite.dylib		libnio.dylib			resources.jar
htmlconverter.jar		libhprof.dylib			libnpt.dylib			rt.jar
images				libinstrument.dylib		libosx.dylib			security
javafx.properties		libj2gss.dylib			libosxapp.dylib			server
javaws.jar			libj2pcsc.dylib			libosxkrb5.dylib		servicetag
jce.jar				libj2pkcs11.dylib		libosxui.dylib			sound.properties
jfr.jar				libjaas_unix.dylib		libprism-es2.dylib		xawt
jfxrt.jar			libjava.dylib			librmi.dylib			zi
```