Remove temporary files when updating library_index.json

[cmaglie]

This is a tentatve fix for #4272 #4332, following @vicnevicne hint.

I'm not able to reproduce the bug, @vicnevicne @MichaelJonker if you can reproduce it, by any chanche, can you try the fix? (ArduinoBot should make a build in a moment...)

[vicnevicne]

@cmaglie I'm confirming the fix is OK. Thanks.
In fact, the issue is easy to reproduce : use the library manager in 1.6.7 (or a version without your fix) and a file "C:\Users\AppData\Local\Arduino15\library_index.json.tmp.gz" gets created but not deleted. Then exit, let one minute pass, restart the same procedure and you'll notice the timestamp of that .gz file does not change (the file is not replaced). So the same file gets unzipped over and over.
With your fixed version, the .gz file is correctly deleted.
Here is the video with 2 successive launches of 1.6.7 and then 2 with your 1.6.8 : http://screencast.com/t/a8hGl2sd1S
So it's all good for me.
@matthijskooijman Could be done indeed, but I guess the separate download/rename/unzip process eases debugging (well, maybe debugging is the reason why the delete was forgotten in the first place).
For me anything goes as long as it works ;-)

[cmaglie]

@vicnevicne
thanks for testing and for the screencast!
It seems that the File.renameTo(..) is not able to replace the destination file on Windows (while it does on linux/unix, that's why I wasn't able to reproduce here).

@matthijskooijman
right, I've added another commit and removed the extra rename. I'll check the result with @vicnevicne's procedure and merge straight away if everything is ok.

[matthijskooijman]

@cmaglie The extra commit looks good, that's exactly what I meant indeed.

[vicnevicne]

@cmaglie : I didn't know renameTo() does overwrite on Unix/Linux, but I know for sure it doesn't on Windows indeed.
The doc confirms that "Many aspects of the behavior of this method are inherently platform-dependent: [...] it might not succeed if a file with the destination abstract pathname already exists" - http://docs.oracle.com/javase/8/docs/api/java/io/File.html#renameTo-java.io.File-

And the improvement following @matthijskooijman 's suggestion is even better. I checked that the downloader also requires the target file to be deleted beforehand, and it's the case (otherwise it appends to it, which gets detected as a corrupt download and forces the fallback to download the non-gz version).

So +1 for the last version, avoiding renameTo() altogether.