Replace deprecated throw with assert in Solidity tests

[TalAter]

Both test/helpers/ReentrancyAttack.sol and test/helpers/ReentrancyMock.sol used throw which has been deprecated since 0.4.13

Replacing them with asserts removes the two ugly error messages that have plagued the tests https://travis-ci.org/OpenZeppelin/zeppelin-solidity/jobs/289039499#L535

[facuspagnuolo]

@TalAter nice catch! would you mind using require instead of assert? Basically require is meant to be used for validations, while assert should be used in order to prevent conditions which should never be possible. I highly recommend this article https://media.consensys.net/when-to-use-revert-assert-and-require-in-solidity-61fb2c0e5a57

[TalAter]

Updated.

Thanks @facuspagnuolo 👍