Make aasm thread-safe. (#442)

Ever since AASM supported multiple named state machines in a single
class, it has accessed and stored its state machines in class variables.
However accesses to the state machines are not thread-safe causing
exceptions when multiple threads attempt to access the same state
machine at the same time during lazy initialization.
Using a Concurrent Map makes calls to state machines, and lazy
initialization thread safe.

aasm.gemspec

@@ -16,6 +16,8 @@ Gem::Specification.new do |s|
   s.platform              = Gem::Platform::RUBY
   s.required_ruby_version = '>= 1.9.3'
 
+  s.add_dependency 'concurrent-ruby', '~> 1.0'
+
   s.add_development_dependency 'rake'
   s.add_development_dependency 'sdoc'
   s.add_development_dependency 'rspec', ">= 3"

lib/aasm/state_machine_store.rb

@@ -1,8 +1,11 @@
+require 'concurrent'
 module AASM
   class StateMachineStore
+    @stores = Concurrent::Map.new
+
     class << self
       def stores
-        @stores ||= {}
+        @stores
       end
 
       # do not overwrite existing state machines, which could have been created by
@@ -38,7 +41,7 @@ module AASM
     end
 
     def initialize
-      @machines = {}
+      @machines = Concurrent::Map.new
     end
 
     def clone