Add recursive binary search for ruby (ch. 1) and improve recursive max for ruby (ch. 4) (#113)

* add recursive binary search for ruby

* account for cases with empty lists and lists with only one item

* use p instead of print to show nil value

* add newline at end of file

04_quicksort/ruby/04_recursive_max.rb

@@ -1,7 +1,8 @@
 def max(list)
-  if list.length == 2
-    # condition ? then : else
-    list[0] > list[1] ? list[0] : list[1]
+  if list.empty?
+    nil
+  elsif list.length == 1
+    numbers[0]
   else
     sub_max = max(list[1..-1])
     list[0] > sub_max ? list[0] : sub_max