Fix bundler dependency resolution during development

This allows bundler to complete gem dependency resolution during
development. This is only relevant for paperclip developers. Please see:
https://github.com/thoughtbot/paperclip/issues/1256 and thanks to
@nkondratyev

paperclip.gemspec

@@ -49,5 +49,5 @@ Gem::Specification.new do |s|
   s.add_development_dependency('rake')
   s.add_development_dependency('fakeweb')
   s.add_development_dependency('railties')
-  s.add_development_dependency('actionmailer')
+  s.add_development_dependency('actionmailer', '>= 3.0.0')
 end