Fix Thread leaking in ThreadLocalVariables

- introduce #bind method to avoid value leaks
- warn on using just #value=
This commit is only viable because we create a JRuby-specific gem build. The conditional statement in the gemspec file is not processed at runtime. It is only processed when the gem is built. Therefore it *should* prevent the `Ref` gem from being installed when using the JRuby-specific build. This has not been tested, however.