Fix memory error in Python2.7 caused by range

Author: karlfloersch

ethereum/chain.py

@@ -1,4 +1,5 @@
 import time
+from builtins import range
 from ethereum import utils
 from ethereum.utils import parse_as_bin, big_endian_to_int
 from ethereum import parse_genesis_declaration
@@ -359,4 +360,3 @@ def db(self):
     @property
     def config(self):
         return self.env.config
-

requirements.txt

@@ -4,6 +4,7 @@ PyYAML
 repoze.lru
 pbkdf2
 pycryptodome>=3.3.1
+future==0.16.0
 scrypt
 rlp>=0.4.7
 https://github.com/ethereum/ethash/tarball/master