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)
```

benchmark/samplers.rb

@@ -86,6 +86,9 @@ def fib(n)
 reporter.measure :exponential_red_black, Metriks::Timer.new(Metriks::ExponentiallyDecayingSample.new(
   Metriks::Histogram::DEFAULT_SAMPLE_SIZE, Metriks::Histogram::DEFAULT_ALPHA, RedBlackTree.new))
 
+reporter.measure :exponential_concurrent_red_black, Metriks::Timer.new(Metriks::ExponentiallyDecayingSample.new(
+  Metriks::Histogram::DEFAULT_SAMPLE_SIZE, Metriks::Histogram::DEFAULT_ALPHA, ConcurrentRedBlackTree.new))
+
 puts "fib(#{fib_times}): #{iter} iterations"
 puts "-" * 50
 

lib/metriks/exponentially_decaying_sample.rb

@@ -7,7 +7,7 @@ class ExponentiallyDecayingSample
     RESCALE_THRESHOLD = 60 * 60 # 1 hour
 
     def initialize(reservoir_size, alpha, values = nil)
-      @values = values || RedBlackTree.new
+      @values = values || ConcurrentRedBlackTree.new
       @count = Atomic.new(0)
       @next_scale_time = Atomic.new(0)
       @alpha = alpha

metriks.gemspec

@@ -41,7 +41,7 @@ Gem::Specification.new do |s|
   ## that are needed for an end user to actually USE your code.
   s.add_dependency('atomic', ["~> 1.0"])
   s.add_dependency('hitimes', [ "~> 1.1"])
-  s.add_dependency('avl_tree', [ "~> 1.1.2" ])
+  s.add_dependency('avl_tree', [ "~> 1.2.0" ])
 
   ## List your development dependencies here. Development dependencies are
   ## those that are only needed during development