Restore back Ruby 2.0 support

Postponing until next major release.
Refs. efff66d

.travis.yml

@@ -27,6 +27,12 @@ before_script:
 
 matrix:
   include:
+    - rvm: 2.0
+      gemfile: gemfiles/rails-4-0.gemfile
+    - rvm: 2.0
+      gemfile: gemfiles/rails-4-1.gemfile
+    - rvm: 2.0
+      gemfile: gemfiles/rails-4-2.gemfile
     - rvm: 2.1.10
       gemfile: gemfiles/rails-4-0.gemfile
     - rvm: 2.1.10

carrierwave.gemspec

@@ -19,7 +19,7 @@ Gem::Specification.new do |s|
   s.require_paths = ["lib"]
   s.licenses = ["MIT"]
 
-  s.required_ruby_version = ">= 2.1.0"
+  s.required_ruby_version = ">= 2.0.0"
 
   s.add_dependency "activesupport", ">= 4.0.0"
   s.add_dependency "activemodel", ">= 4.0.0"

lib/carrierwave/uploader.rb

@@ -43,6 +43,15 @@ module Uploader
     class Base
       attr_reader :file
 
+      ##
+      # Workaround for class_attribute malfunction when used with Module#prepend
+      #
+      if RUBY_VERSION < '2.1.0'
+        def self.singleton_class?
+          !ancestors.include? self
+        end
+      end
+
       include CarrierWave::Uploader::Configuration
       include CarrierWave::Uploader::Callbacks
       include CarrierWave::Uploader::Proxy