Fix rlp version

[matsulib]

fix #253

This patch adapts the rlp version to latest devp2p.
https://github.com/ethereum/pydevp2p/blob/v0.9.2/requirements.txt

[matsulib]

Oh, sorry. In Travis CI, the coverage test resulted in errors.
All errors are same, like this.

_________________ ERROR collecting pyethapp/tests/test_app.py __________________
ImportError while importing test module '/home/travis/build/ethereum/pyethapp/pyethapp/tests/test_app.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
pyethapp/tests/test_app.py:3: in <module>
    from pyethapp import app
pyethapp/app.py:28: in <module>
    from console_service import Console
pyethapp/console_service.py:27: in <module>
    from pyethapp.rpc_client import ABIContract
pyethapp/rpc_client.py:14: in <module>
    from tinyrpc.transports.http import HttpPostClientTransport
build/bdist.linux-x86_64/egg/tinyrpc/transports/http.py:5: in <module>
    ???
E   ImportError: No module named geventwebsocket

There is a missing dependency because latest tinyrpc uses geventwebsocket in a specific way.
To solve the dependency, it is necessary to specify [websocket] when installing tinyrpc.

pip install tinyrpc[websocket]

I will fix this soon.