Rollback backwards incompatible change

We upgraded the minimum required Ruby version to not include end-of-life
Rubies, but forgot to take into account that it's a backwards
incompatible change.

In this commit we rollback that change, allowing paperclip ~> 4.2.4 to
run on 1.9.2 again.

[fixes #1902]

paperclip.gemspec

@@ -20,7 +20,7 @@ Gem::Specification.new do |s|
   s.require_paths = ["lib"]
 
   s.requirements << "ImageMagick"
-  s.required_ruby_version = ">= 2.0.0"
+  s.required_ruby_version = ">= 1.9.2"
 
   s.add_dependency('activemodel', '>= 3.2.0')
   s.add_dependency('activesupport', '>= 3.2.0')