Add serpent to test requirements
Add serpent to test requirements

test_eth_service uses tester, which in turn needs serpent

gsalgado/pyethapp,changwu-tw/pyethapp,changwu-tw/pyethapp,RomanZacharia/pyethapp,RomanZacharia/pyethapp,d-das/pyethapp,vaporry/pyethapp,ethereum/pyethapp,ethereum/pyethapp,gsalgado/pyethapp