Fix compiler version

[ofirbarak]

While trying to running tests, it encounter a problem with the compiler version:

flash-lending/contracts/interface/IBank.sol:19:1: SyntaxError: Source file requires different compiler version (current compiler is 0.4.26+commit.4563c3fc.Emscripten.clang - note that nightly builds are considered to be strictly less than the released version
pragma solidity 0.4.24;
^---------------------^

Solved by fix compiler versions in all files to 0.4.26