fixed Ubuntu12.04 compile error
fixed Ubuntu12.04 compile error

debris/solidity,chriseth/solidity,arkpar/solidity,shahankhatch/solidity,ruchevits/solidity,ruchevits/solidity,ruchevits/solidity,gluk256/solidity,ruchevits/solidity,LianaHus/solidity,douglas-larocca/solidity,ethers/solidity,chriseth/solidity-doc-test,vaporry/solidity