Add Go modules support but keep dep support

[liamsi]

• remove dep artifacts
• remove traces of dep from make file: go build will handle those for us
• update circelci config to go1.12
• make sure all tests pass (fix #256 supersedes/closes #257)
• replace metalinter with golangci-lint (much faster)
• add back (updated) Gopkg.toml / .lock

cc @jackzampolin

[liamsi]

Interestingly, some combination of updated dependencies reveals a failing test: https://circleci.com/gh/tendermint/go-amino/521?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link (can be reproduced locally, there is an error where a panic is expected -> testify let this pass unnoticed ...)

[liamsi]

Meh, I've opened https://github.com/tendermint/go-amino/issues/256 It's probably worth to fix the tests here directly.

[liamsi]

Thanks @melekes!