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

spec/paperclip/storage/fog_spec.rb

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