[solc] Need to upgrade compiler version

[kazuakiishiguro]

CompileError: maci-contracts/sol/MACI.sol:3:1: ParserError: Source file requires different compiler version (current compiler is                              
 0.5.16+commit.9c3226ce.Emscripten.clang - note that nightly builds are considered to be strictly less than the released version                               
pragma solidity ^0.6.12;                                                                                                                                      
^----------------------^                                                                                                                                      
) 

current compiler version 0.5.16

[kazuakiishiguro]

Implemented at cbcf302