Update nix tarball and minimum solidity version

Fixes #3.

contracts/Foo.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.4.0;
+pragma solidity ^0.4.24;
 
 contract Foo {
     function Foo() public {

contracts/MyThrow.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.4.0;
+pragma solidity ^0.4.24;
 
 contract MyThrow {
     function MyThrow() public {

contracts/PisaChannel.sol

@@ -1,8 +1,8 @@
-// Author: Patrick McCorry 
-// State channel construction from https://eprint.iacr.org/2018/582 
-// Can be used for any state channel application - assuming it implements the transition function 
+// Author: Patrick McCorry
+// State channel construction from https://eprint.iacr.org/2018/582
+// Can be used for any state channel application - assuming it implements the transition function
 
-pragma solidity ^0.4.7;
+pragma solidity ^0.4.24;
 
 interface Application {
 

contracts/PreimageManager.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.4.20;
+pragma solidity ^0.4.24;
 
 contract PreimageManager {
     mapping ( bytes32 => uint ) timestamp;

contracts/SpritesRegistry.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.4.20;
+pragma solidity ^0.4.24;
 // XXX enable returning structs from internal functions
 pragma experimental ABIEncoderV2;
 

contracts/Test.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.4.0;
+pragma solidity ^0.4.24;
 pragma experimental ABIEncoderV2;
 
 contract Test {

contracts/contractPay.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.4.20;
+pragma solidity ^0.4.24;
 
 // Note: Initial version does NOT support concurrent conditional payments!
 

contracts/contractSprite.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.4.20;
+pragma solidity ^0.4.24;
 
 // External interface
 interface PreimageManager {

contracts/dappsys.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.4.19;
+pragma solidity ^0.4.24;
 
 /// math.sol -- mixin for inline numerical wizardry
 

shell.nix

@@ -1,8 +1,7 @@
 with import (
   builtins.fetchTarball {
-   # Tue Apr 17 15:50:00 HKT 2018 pipenv 11.10.0
-    url = "https://github.com/sveitser/nixpkgs/archive/835e86adfd33b10858751bf329a43e67ae89761f.tar.gz";
-    sha256 = "129wvirbp43n8hkz0lywzhdv1sa2cjxs4lwr7wasq7p3sa2zms6b";
+    url = "https://releases.nixos.org/nixpkgs/nixpkgs-18.09pre147700.03e47c388ac/nixexprs.tar.xz";
+    sha256 = "06prf50w9w5qkrjhxgj7dkwwxfanh9akv3qfb6ibh8mqp1jmnwqm";
   }
 ) {};