Fix generic types issue in the upgrades functions

[jagodarybacka]

Reference #46 (comment)

There is a problem with using typechain's contract interfaces like upgradeProxy<ContractTypeFromTypechain>() - the types don't match because typechain is creating types based on the ethers' BaseContract type but OpenZeppelin's upgradeProxy and deployProxy are returning ethers' Contract type - these are not interchangeable.

OpenZeppelin/openzeppelin-upgrades#535 would probably fix this issue but is not merged yet