Allow `minimumDaoBounty` >= 0

Kei from Gnosis was interested in deploying a DAO with a minimumDaoBounty = 0, the voting machine configuration is pasted below. It seems strange that the smart contracts would prohibit this, and it should be something that's at the user's discretion to set properly as long as it's >= 0.

Requested Code Change:
`require(_params[8] >= 0, "minimumDaoBounty should be >= 0");`

Original Code:
https://github.com/daostack/infra/blob/a09e9a524fbacc9fb39d5d1de8986dcf45c4889c/contracts/votingMachines/GenesisProtocolLogic.sol#L263

Config:
```json
{
"queuedVoteRequiredPercentage": 20,
"queuedVotePeriodLimit": 604800,
"thresholdConst": 1001,
"proposingRepReward": 10,
"minimumDaoBounty": 0,
"boostedVotePeriodLimit": 172800,
"daoBountyConst": 3,
"activationTime": 0,
"preBoostedVotePeriodLimit": 43200,
"quietEndingPeriod": 43200,
"voteOnBehalf": "0x0000000000000000000000000000000000000000",
"votersReputationLossRatio": 1
}
```