packaging: Subversion 1.8 -> 1.9

Updating to the current version from nixpkgs.

default.nix

@@ -16,8 +16,7 b' let'
   pkgs = pkgs_.overridePackages (self: super: {
     # Override subversion derivation to
     #  - activate python bindings
-    #  - set version to 1.8
-    subversion = super.subversion18.override {
+    subversion = super.subversion.override {
        httpSupport = true;
        pythonBindings = true;
        python = self.python27Packages.python;