remove python2.6 for travis
remove python2.6 for travis

ethereum/pyethereum,pipermerriam/pyethereum,shahankhatch/pyethereum,ethermarket/pyethereum,ethers/pyethereum,karlfloersch/pyethereum,ebuchman/daoist_protocol,vaporry/pyethereum,ebuchman/daoist_protocol,karlfloersch/pyethereum,harlantwood/pyethereum,shahankhatch/pyethereum,ddworken/pyethereum,pipermerriam/pyethereum,ethereum/pyethereum,holiman/pyethereum,jnnk/pyethereum,inzem77/pyethereum,ckeenan/pyethereum