Move Timecop gem from Gemfile into gemspec

Gemfile

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

paperclip.gemspec

@@ -47,4 +47,5 @@ Gem::Specification.new do |s|
   s.add_development_dependency('railties')
   s.add_development_dependency('actionmailer', '>= 3.0.0')
   s.add_development_dependency('generator_spec')
+  s.add_development_dependency('timecop')
 end