Elixir and rebar aren't Linux-specific tools

Widen platform support to all unix

pkgs/development/interpreters/elixir/default.nix

···
     '';
 
     license = licenses.epl10;
-    platforms = platforms.linux;
+    platforms = platforms.unix;
     maintainers = [ maintainers.the-kenny ];
   };
 }

pkgs/development/tools/build-managers/rebar/default.nix

···
       variety of locations (git, hg, etc).
       '';
 
-    platforms = stdenv.lib.platforms.linux;
+    platforms = stdenv.lib.platforms.unix;
     maintainers = [ stdenv.lib.maintainers.the-kenny ];
   };
 }