Ensure redis polling updates are done in a transaction (#30)

Without this, there is an interval during which flag values are missing.

Author: ashanbrown

lib/ldclient-rb/redis_feature_store.rb

@@ -158,8 +158,10 @@ def delete(key, version)
     def init(fs)
       @cache.clear
       with_connection do |redis|
-        redis.del(@features_key)
-        fs.each { |k, f| put_redis_and_cache(redis, k, f) }
+        redis.multi do |multi|
+          redis.del(@features_key)
+          fs.each { |k, f| put_redis_and_cache(multi, k, f) }
+        end
       end
       put_cache(INIT_KEY, true)
       @logger.info("RedisFeatureStore: initialized with #{fs.count} feature flags")