pantheon: use latest vala

elementary OS's ecosystem is curated around Ubuntu's LTS releases.
This means the development platform for their curated applications
always includes a LTS version of vala (in 18.04 it's 0.40).
Because of how vala development works it suspect some of these
applications to have serious issues if complied with the latest vala.
However in the past year or so, for Pantheon at least, I don't think
their applications will have much issues with latest vala, and if there
is I don't think they'd be difficult to fix. In this single regard they've
become more responsive since their preferred language is vala.

As for the curated applications I have less of this confidence in.
So I'd have to be accept less applications, but that's something
I'm willing to compromise on. And this is easily reversible or
could be done on a per-application basis. And nix already makes
this trivial.

pkgs/tools/misc/hashit/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchFromGitHub, meson, ninja, pkgconfig, pantheon, python3, libgee, gtk3, desktop-file-utils, wrapGAppsHook }:
+{ stdenv, fetchFromGitHub, meson, ninja, pkgconfig, vala, pantheon, python3, libgee, gtk3, desktop-file-utils, wrapGAppsHook }:
 
 stdenv.mkDerivation rec {
   pname = "hashit";
@@ -15,7 +15,7 @@ stdenv.mkDerivation rec {
     desktop-file-utils
     meson
     ninja
-    pantheon.vala
+    vala
     pkgconfig
     python3
     wrapGAppsHook