Replace sneakers with something maintained

[lwrubel]

sneakers has rake pinned to version 12 and this is incompatible with other current Rails dependencies. rake version 13 is needed to run bin/rails assets:clobber with H2's dependencies.

We need to consider alternatives to sneakers, which is not currently maintained: see jondot/sneakers#452

[michaelklishin]

Sneakers maintainers have moved it to https://github.com/ruby-amqp/kicks. I suggest that you switch to it and submit any PRs you find crucial to include there.

We plan to produce a RubyGems release reasonably soon, starting with a few previews.