Update Ruby and Rails dependencies in gemspec

Pairing with @kitop

paperclip.gemspec

@@ -22,15 +22,15 @@ Gem::Specification.new do |s|
   end
 
   s.requirements << "ImageMagick"
-  s.required_ruby_version = ">= 1.9.2"
+  s.required_ruby_version = ">= 2.1.0"
 
-  s.add_dependency('activemodel', '>= 3.2.0')
-  s.add_dependency('activesupport', '>= 3.2.0')
+  s.add_dependency('activemodel', '>= 4.2.0')
+  s.add_dependency('activesupport', '>= 4.2.0')
   s.add_dependency('cocaine', '~> 0.5.5')
   s.add_dependency('mime-types')
   s.add_dependency('mimemagic', '~> 0.3.0')
 
-  s.add_development_dependency('activerecord', '>= 3.2.0')
+  s.add_development_dependency('activerecord', '>= 4.2.0')
   s.add_development_dependency('shoulda')
   s.add_development_dependency('rspec', '~> 3.0')
   s.add_development_dependency('appraisal')
@@ -49,7 +49,7 @@ 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', '>= 3.2.0')
+  s.add_development_dependency('actionmailer', '>= 4.2.0')
   s.add_development_dependency('generator_spec')
   s.add_development_dependency('timecop')
 end