Update pyethereum version in setup.py

[Varunram]

The current bug in the Travis setup reads error: Setup script exited with error in ethereum setup command: 'tests_require' must be a string or list of strings containing valid project/version requirement specifiers; Unordered types are not allowed and a suggested fix is to upgrade pyetheruem. Is there any specific reason as to why we used a specific version of pyethereum in setup.py? If not, I can put in a PR for this issue. Thanks!

[hwwhww]

I just merged #53 and fixed CI.

The reasons that we use the specific "commit" of pyethereum are:

1. sharding repo also depends on Vyper specific "commit", and it requires specific pyethereum while we want to use pyethereum with Python 3.6 supports.
2. pyethereum and Vyper are in rapid development, and sometimes, it's not backward compatible. So we have to lock the dependencies in our development.

[Varunram]

Thanks!