only create/allocate GitHub CIDR object once

lib/travis/api/attack.rb

@@ -32,13 +32,15 @@ class Rack::Attack
     "/auth/post_message/iframe"
   ]
 
+  GITHUB_CIDR = NetAddr::CIDR.create('192.30.252.0/22')
+
   whitelist('safelist build status images') do |request|
     /\.(png|svg)$/.match(request.path)
   end
 
   # https://help.github.com/articles/what-ip-addresses-does-github-use-that-i-should-whitelist/
   whitelist('safelist anything coming from github') do |request|
-    request.ip && NetAddr::CIDR.create('192.30.252.0/22').contains?(request.ip)
+    request.ip && GITHUB_CIDR.contains?(request.ip)
   end
 
   ####