Added update_url to json to avoid corrupted extension message on Chrome 63+

[BoxOfSnoo]

I mentioned this in an issue. Here's a simple fix

[Cortys]

Hi! Thanks for the PR.

I'm not able to reproduce the issue this fixes, so I'm currently holding back on merging. As far as I know, the corruption warning only occurs when installing crx files. Since this extension is not released as a crx this does not apply though.

Could you describe how you are able produce this issue?

[sgbeal]

i was getting this error at some point, too (installing "unpacked" from the source tree), but it hasn't happened to me in any recent updates, so it seems that that may have been a short-lived bug in Chrome/Chromium (it's conceivable that that version/those versions may still be the latest versions on some Linux distros).

[BoxOfSnoo]

I think @sgbeal is right. I was getting this even when using "unpacked" with Chrome 63 back when I created the PR. It doesn't seem to happen now.