Fix standard ruby style issues

Gemfile

@@ -10,4 +10,3 @@ gem "rake", "~> 13.0"
 gem "rspec", "~> 3.0"
 
 gem "standard", "~> 1.3"
-

lib/rex/limit.rb

@@ -14,7 +14,7 @@ module Rex
     def pop_first_order
       order = @first_order
       @first_order = @first_order.next_order
-      if @first_order == nil
+      if @first_order.nil?
         @last_order = nil
       end
 
@@ -25,11 +25,10 @@ module Rex
     def add_order(order)
       if empty?
         @first_order = order
-        @last_order = order
       else
         @last_order.next_order = order
-        @last_order = order
       end
+      @last_order = order
       @order_count += 1
     end
 

lib/rex/order_book.rb

@@ -2,7 +2,7 @@ require "rbtree"
 
 module Rex
   class OrderBook
-    def initialize()
+    def initialize
       @sell_side = tree_with_limit_default
       @buy_side = tree_with_limit_default
     end
@@ -17,7 +17,6 @@ module Rex
       side[order.price].add_order(order)
     end
 
-
     def cancel_order(order_id)
     end