// Sends increateTime instruction, mines block and returns a Promise.
function increaseTime (addSeconds) {
    web3.currentProvider.sendAsync({
      jsonrpc: '2.0',
      method: 'evm_increaseTime',
      params: [addSeconds],
      id: new Date().getSeconds()
    }, (err) => {
      if (!err) {
        web3.currentProvider.send({
          jsonrpc: '2.0',
          method: 'evm_mine',
          params: [],
          id: new Date().getSeconds()
        });
      }
    });
  }