Adding pythonPackags.pandas to binary cache. Python packages are

not part of nixpkgs/nixos jobsets in 16.03+ since ccd1029f58. Until
it gets added again, adding some python packages that take really
long to build.

(cherry picked from commit 713c24056397fef46717d2f0eae1940f348941e6)

pkgs/top-level/release.nix

@@ -259,7 +259,9 @@ let
 
       perlPackages = { };
 
-      pythonPackages = { };
+      pythonPackages = {
+        pandas = unix;
+      };
       python2Packages = { };
       python27Packages = { };
       python3Packages = { };