add "pragma solidity ^0.4.0;" to code examples

docs/security-considerations.rst

@@ -51,6 +51,8 @@ complete contract):
 
 ::
 
+    pragma solidity ^0.4.0;
+
   // THIS CONTRACT CONTAINS A BUG - DO NOT USE
   contract Fund {
       /// Mapping of ether shares of the contract.
@@ -73,6 +75,8 @@ outlined further below:
 
 ::
 
+    pragma solidity ^0.4.0;
+
   contract Fund {
       /// Mapping of ether shares of the contract.
       mapping(address => uint) shares;
@@ -149,6 +153,8 @@ Never use tx.origin for authorization. Let's say you have a wallet contract like
 
 ::
 
+    pragma solidity ^0.4.0;
+
     contract TxUserWallet {
         address owner;
 
@@ -166,6 +172,8 @@ Now someone tricks you into sending ether to the address of this attack wallet:
 
 ::
 
+    pragma solidity ^0.4.0;
+
     contract TxAttackWallet {
         address owner;