Require 'concurrent' only when needed

This helps reduce the memory overhead of Bugsnag when session tracking
isn't enabled by ~4 MiB

lib/bugsnag/session_tracker.rb

@@ -1,11 +1,9 @@
 require 'thread'
 require 'time'
 require 'securerandom'
-require 'concurrent'
 
 module Bugsnag
   class SessionTracker
-
     THREAD_SESSION = "bugsnag_session"
     SESSION_PAYLOAD_VERSION = "1.0"
     MUTEX = Mutex.new
@@ -27,6 +25,8 @@ def self.get_current_session
     ##
     # Initializes the session tracker.
     def initialize
+      require 'concurrent'
+
       @session_counts = Concurrent::Hash.new(0)
     end
 
@@ -139,4 +139,4 @@ def deliver(session_payload)
       Bugsnag::Delivery[Bugsnag.configuration.delivery_method].deliver(Bugsnag.configuration.session_endpoint, payload, Bugsnag.configuration, options)
     end
   end
-end
+end