Cache hash should depend on Favicons version

[wclr]

Is it possible to hash individual files?
The case why:

There is an issue itgalaxy/favicons#253

With different favicons version, the same source file is processed differently. If we hash only by source file we have a situation with a stale cache on a client.

Or icons-[hash] should also depend on exact favicons version.

[brunocodutra]

Or icons-[hash] should also depend on exact favicons version

That's actually a good point, I think it should indeed.

[wclr]

And maybe svg2png or phantomjs =)

[brunocodutra]

I'm not sure about these, since they are transitive dependencies that we don't control.

[brunocodutra]

I think a compromise we can make to improve the situation is to have [hash] depend on the version of webapp-webpack-plugin.