tor-browser-bundle: update license to free

The Tor Browser Bundle is free software under various licenses:

> Can I distribute Tor?
> 
> Yes.
> 
> The Tor software is free software. This means we give you the rights
> to redistribute the Tor software, either modified or unmodified,
> either for a fee or gratis. You don't have to ask us for specific
> permission.
> 
> However, if you want to redistribute the Tor software you must follow
> our LICENSE. Essentially this means that you need to include our
> LICENSE file along with whatever part of the Tor software you're
> distributing.
> 
> Most people who ask us this question don't want to distribute just the
> Tor software, though. They want to distribute the Tor Browser. This
> includes Firefox Extended Support Release, and the NoScript and
> HTTPS-Everywhere extensions. You will need to follow the license for
> those programs as well. Both of those Firefox extensions are
> distributed under the GNU General Public License, while Firefox ESR is
> released under the Mozilla Public License. The simplest way to obey
> their licenses is to include the source code for these programs
> everywhere you include the bundles themselves. 

(https://www.torproject.org/docs/faq.html.en#DistributingTor)

tor-browser-bundle-bin is already marked as licenses.free, so it doesn't
really make sense that this one is marked as unfree.

pkgs/applications/networking/browsers/tor-browser-bundle/default.nix

@@ -340,7 +340,7 @@ stdenv.mkDerivation rec {
   meta = with stdenv.lib; {
     description = "An unofficial version of the tor browser bundle, built from source";
     homepage = https://torproject.org/;
-    license = licenses.unfreeRedistributable; # TODO: check this
+    license = licenses.free;
     platforms = [ "x86_64-linux" ];
     hydraPlatforms = [ ];
     maintainers = with maintainers; [ joachifm ];