Fix Thread leaking in ThreadLocalVariables

- introduce #bind method to avoid value leaks
- warn on using just #value=
Fixing ThreadLocalVar to use 'ref' gem

ThreadLocalVar needs to be required manually.
Added Ref gem dependency on non-JRuby systems.

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.
Removed Ref from the Gemfile.
Merge pull request #164 from ruby-concurrency/thread-local-var

Fix Thread leaking in ThreadLocalVariables

Tested the exclusion of the `Ref` gem in the pre-compiled Java build (using our automated build process) and it worked as intended. The `Ref` gem will not be installed under JRuby when using the precompiled Java build.