Soften checks for Ruby head in CI

There have been several instances ([the most recent one](https://github.com/bytecodealliance/wasmtime-rb/pull/285)) where Ruby's head version fails in CI. This is not always ideal, given that most of the time the failures are not directly related to changes in a particular pull-request, instead the failures are related to changes in Ruby's latest version. If there's interest in keeping checks against Ruby's head version, would it be reasonable to "soften" these checks and emit a warning instead of an error?