Bump up avl_gem and use ConcurrentRedBlackTree

It's slightly slower according to benchmark we have for samplers but
we get thread safety.

```
    ~/projects/metriks concurrent-redbacktree * be ruby ./benchmark/samplers.rb
    Resolving dependencies...
    The Gemfile's dependencies are satisfied
    fib(10): 100000 iterations
    --------------------------------------------------
                      plain: 0.863473 secs 0.000009 secs/call
                    uniform: 2.034218 secs 0.000020 secs/call
                              - 135.6% slower than plain (0.000012 secs/call)
            exponential_avl: 2.227544 secs 0.000022 secs/call
                              - 9.5% slower than uniform (0.000002 secs/call)
                              - 158.0% slower than plain (0.000014 secs/call)
      exponential_red_black: 2.273125 secs 0.000023 secs/call
                              - 2.0% slower than exponential_avl (0.000000 secs/call)
                              - 163.3% slower than plain (0.000014 secs/call)
    exponential_concurrent_red_black: 2.433853 secs 0.000024 secs/call
                              - 7.1% slower than exponential_red_black (0.000002 secs/call)
                              - 181.9% slower than plain (0.000016 secs/call)
```
Merge pull request #49 from strzalek/concurrent-redblacktree

Bump up avl_gem and use ConcurrentRedBlackTree