fix CI

[nyurik]

This fixes issues at the https://github.com/rust-brotli/rust-brotli/pull/43/checks github CI builds. If this project is to be successful, we must start using CI for every PR as a guarantee of code quality.

• Bump version to 6.0 (per semver check)
• Remove unused SIMD use statements
• hide a few warnings - these are TODOs, and should be fixed in separate PRs
• do NOT build SIMD as part of MSRV -- doesn't make any sense to combine nightly with MSRV
• set edition explicitly to 2015. We should bump it soon - it makes no sense to use an older edition when we have MSRV