SPDX license identifiers missing

[wbt]

As of today (Solidity 0.6.8, from this PR), Solidity issues warnings on Migrations.sol (and other) contracts for lack of an "SPDX-License-Identfier" code comment at the top of the file.

A proposed fix would be to add //SPDX-License-Identifier: MIT to the start of Migrations.sol and every other contract file, in this and all the other boxes. I would rather not go through every single repo in truffle-box in addition to the main Truffle repo to add this same Issue.