Allow higher rlp version
Allow higher rlp version

This is out of sync with `pyethapp`.

shahankhatch/pyethereum,ethereum/pyethereum,pipermerriam/pyethereum,pipermerriam/pyethereum,karlfloersch/pyethereum,ethereum/pyethereum,karlfloersch/pyethereum,shahankhatch/pyethereum