Reorder type so the ssz serialization matches other uses

There is an order based on the Vyper deposit contract which should be maintained
here. There is also a reference to it when processing `Deposit` messages.

This commit corrects the order here so all serializations will match.

specs/core/0_beacon-chain.md

@@ -368,12 +368,12 @@ Unless otherwise indicated, code appearing in `this style` is to be interpreted
 
 ```python
 {
-    # Deposit input
-    'deposit_input': DepositInput,
     # Amount in Gwei
     'amount': 'uint64',
     # Timestamp from deposit contract
     'timestamp': 'uint64',
+    # Deposit input
+    'deposit_input': DepositInput,
 }
 ```