(Fix) Set exact repo tree for xDai since poa-chain-spec/contracts.json changed

[varasev]

• (Mandatory) Description Fixes https://github.com/poanetwork/poa-dapps-validators/issues/120. We replaced https://github.com/poanetwork/poa-chain-spec/blob/dai/contracts.json and https://github.com/poanetwork/poa-chain-spec/tree/dai/abis today in https://github.com/poanetwork/poa-chain-spec/pull/140, but DApp uses the previous versions from this tree: https://github.com/poanetwork/poa-chain-spec/tree/ce2c77256f0d37fc48baa9b6cab806261d034785. This PR sets a patch to use the correct tree for dai branch. After POSDAO is activated, the DApp should stop supporting xDai because the current POA consensus contracts will stop to be used on xDai (a PR for removing xDai from the DApp will be opened accordingly).
• (Mandatory) What is it: (Fix), (Feature), or (Refactor) (Fix)