upgrade to solidity 0.6.8

[ralph-pichler]

This upgrades all contracts and tools for solidity 0.6.8.

Changes:

• all tools have been update to versions compatible with 0.6.8.
• disabling mark-callable-contracts in solhint no longer needed
• compiler-version solhint check had to be disabled as it expects solidity 0.5.
• last remnants of ethlint / solium have been removed
• Uses ERC20PresetMinterPauser as a test token as ERC20Mintable has been removed from openzeppelin.
• The import for ERC20PresetMinterPauser (to ensure its available during tests) now happens in Migrations.sol so that no test code appears in the main sol files.
• To ensure binding for ERC20PresetMinterPauser are present for go-side testing in the bindings, go bindings for Migrations are also generated.

[Eknir]

Looks good. Thanks for doing this maintenance.