Remove support for Rails <4.0 and Ruby <1.9.3

Rails 3.2 has been unsupported since Tolk 1.4.

Gemfile

@@ -3,9 +3,3 @@ source "http://rubygems.org"
 gemspec
 
 gem "rails", ENV['RAILS_VERSION']
-
-group 'development' do
-  if RUBY_VERSION < '1.9'
-    gem "ruby-debug", ">= 0.10.3"
-  end
-end

lib/tolk/engine.rb

@@ -7,10 +7,8 @@ class Engine < Rails::Engine
 
     isolate_namespace Tolk
 
-    if Rails.version >= '3.1'
-      initializer :assets do |app|
-        app.config.assets.precompile += ['tolk/libraries.js']
-      end
+    initializer :assets do |app|
+      app.config.assets.precompile += ['tolk/libraries.js']
     end
   end
 end

tolk.gemspec

@@ -14,6 +14,9 @@ Gem::Specification.new do |s|
 
   s.platform = Gem::Platform::RUBY
 
+  s.required_ruby_version = '>= 1.9.3'
+
+  s.add_runtime_dependency 'rails', '~> 4.0.0'
   s.add_runtime_dependency 'will_paginate'
   s.add_runtime_dependency 'safe_yaml', ">= 0.8.6"