Add gaslimit upper bound of 2**63-1 EIP 106

For reference:
- ethereum/EIPs#92
- ethereum/EIPs#106

ethereum/blocks.py

@@ -446,6 +446,9 @@ def __init__(self, header, transaction_list=[], uncles=[], env=None,
             if self.timestamp >= 2 ** 256:
                 raise ValueError("Timestamp waaaaaaaaaaayy too large")
 
+        if self.gas_limit > 2 ** 63 - 1:
+            raise ValueError("Block's gaslimit went too high!")
+
         for uncle in uncles:
             assert isinstance(uncle, BlockHeader)