Migrate to truffle framework

This is a maintenance-only release (no contracts are upgraded).

It includes the migration from etherlime to the truffle framework used for compilation, testing and deployment of contracts. Additionally a number of truffle extensions have been connected and provide code coverage, gas cost reporting and Etherscan verification. #146

A new script is added to the nightly build that checks all tokens in the TokenPriceRegistry for ERC20 compliance using the slither
https://github.com/crytic/slither/wiki/ERC-Conformance tool. #174

Special thanks ❤️ to
@montyly from Trail of Bits for his continuing support on getting the most of slither
@cgewecke for his code-coverage custom plugin contribution in #177
@rkalis for his support in getting the etherscan verification plugin working rkalis/truffle-plugin-verify#46
@gnidan , @eggplantzzz and the truffle and ganache-cli teams for their continuing support