Fix intermittently failing spec

* `fog_spec.rb:328` fails intermittently due to a race condition
* Use [TimeCop][1] gem to freeze time during the spec
* Extract magic number
* Break long line

[1]: https://github.com/travisjeffery/timecop

![TimeCop
Poster](http://www.deconstructingmovies.com/wp-content/uploads/2014/03/timecop-poster.jpg)

Gemfile

@@ -17,4 +17,5 @@ gem 'pry'
 group :development, :test do
   gem 'mime-types', '~> 1.16'
   gem 'builder', '~> 3.1.4'
+  gem 'timecop', '~> 0.7.1'
 end

spec/paperclip/storage/fog_spec.rb

 require 'spec_helper'
 require 'fog'
+require 'timecop'
 
 describe Paperclip::Storage::Fog do
   context "" do
@@ -319,11 +320,15 @@ describe Paperclip::Storage::Fog do
 
       context "generating an expiring url" do
         it "generates the same url when using Times and Integer offsets" do
-          rebuild_model(@options)
-          dummy = Dummy.new
-          dummy.avatar = StringIO.new('.')
-
-          assert_equal dummy.avatar.expiring_url(1234), dummy.avatar.expiring_url(Time.now + 1234)
+          Timecop.freeze do
+            offset = 1234
+            rebuild_model(@options)
+            dummy = Dummy.new
+            dummy.avatar = StringIO.new('.')
+
+            assert_equal dummy.avatar.expiring_url(offset),
+              dummy.avatar.expiring_url(Time.now + offset )
+          end
         end
 
         it 'matches the default url if there is no assignment' do