Fix payment splitter contract calls with updated ethersjs

[elv-serban]

Due to elv-client-js upgrading ethers to version 5 I had to update some of the calls for ambiguous method names.

[elv-preethi]

looks good! Tested payment_contract_show method, doesn't return any error.

Also, this doc can be helpful: https://docs.ethers.org/v5/migration/ethers-v4/