bump tahoe-lafs to latest version of integration/storage-economics

tahoe-lafs.nix

@@ -12,8 +12,8 @@ python.pkgs.buildPythonPackage rec {
     repo = "tahoe-lafs";
     # HEAD of an integration branch for all of the storage plugin stuff.  Last
     # updated August 23 2019.
-    rev = "d4b5de2e08e26ad2cc14265a5993be2ecc791d5b";
-    sha256 = "1l2da13w43zzwr1z262zhhq4hq3sha4zrxp7d46zmjn4ya0ixksf";
+    rev = "8c1f536ba4fbc01f3bc5f08412edbefc56ff7037";
+    sha256 = "17d7pkbsgss3rhqf7ac7ylzbddi555rnkzz48zjqwq1zx1z2jhy6";
   };
 
   postPatch = ''