Move require "big" to relevant util file

src/util/numeric_sort.cr

@@ -2,6 +2,8 @@
 # Used to sort the images files inside the archives
 # https://github.com/hkalexling/Mango/issues/12
 
+require "big"
+
 def is_numeric(str)
   /^\d+/.match(str) != nil
 end

src/util/util.cr

@@ -1,5 +1,3 @@
-require "big"
-
 IMGS_PER_PAGE     = 5
 UPLOAD_URL_PREFIX = "/uploads"
 STATIC_DIRS       = ["/css", "/js", "/img", "/favicon.ico"]