chore(nix): update version and vendor hash (#859)

Author: ctn-malone

default.nix

@@ -2,11 +2,11 @@
 
 pkgs.buildGoModule rec {
   pname = "gum";
-  version = "0.15.0";
+  version = "0.15.2";
 
   src = ./.;
 
-  vendorHash = "sha256-i/KBe41ufYA+tqnB5LCC1geIc2Jnh97JLXcXfBgxdM4=";
+  vendorHash = "sha256-TK2Fc4bTkiSpyYrg4dJOzamEnii03P7kyHZdah9izqY=";
 
   ldflags = [ "-s" "-w" "-X=main.Version=${version}" ];
 }