dependencies: Adding Bower out of nixpkgs

Usually bower would be installed globally, so it seems to be appropriate to use
the one out of the nixpkgs so that we don't have to pin a specific version
ourselves.

default.nix

@@ -98,6 +98,7 b' let'
       buildInputs =
         attrs.buildInputs ++
         (with self; [
+          pkgs.nodePackages.bower
           pkgs.nodePackages.grunt-cli
           pkgs.subversion
           pytest-catchlog