Add missing dependency (PyYAML) to requirements.txt
Add missing dependency (PyYAML) to requirements.txt

pipermerriam/pyethereum,karlfloersch/pyethereum,ckeenan/pyethereum,vaporry/pyethereum,karlfloersch/pyethereum,inzem77/pyethereum,harlantwood/pyethereum,shahankhatch/pyethereum,ddworken/pyethereum,ethereum/pyethereum,ethereum/pyethereum,shahankhatch/pyethereum,pipermerriam/pyethereum,holiman/pyethereum