Release WebExtensions version

[ExE-Boss]

Since Firefox 57 is now officially out, it might be a good idea to release the WebExtensions version update (maybe also change strict_min_version to 57.0a1, so that those on ESR will get to keep the legacy version, which also supports the chrome:// and about: schemes, as can be seen in #3).

[Ede123]

I updated the AMO listing and the update was just approved.

Please test if it actually works as I don't (and probably never will) have Firefox 57.

Thanks again for the update!

[ExE-Boss]

The WebExtension works fine on my end, with the limitations of #3 of course.