feature: Change to a Rails 4 ActiveModel validator.

Rails 4 introduced ActiveModel, ActiveModel::Validations and
ActiveModel::EachValidator.

Instead of injecting ourselves into ActiveRecord::Base, we will be
automatically picked up. This fixes KimNorgaard#1.

We can now also be used on ActiveModel as well as ActiveRecord. This
simplifies testing.

Also
* rspec-rails was dropped becasue it seems to want all of Rails loaded.
  We only needed one convenience method, errors_on. It has been copied into our test model.
* Bumped our runtime dependency to activemodel >= 4.0. Dropped the others.
* Dropped the development dependency on all of Rails. We only need ActiveModel.

README.rdoc

@@ -5,7 +5,7 @@
 
 == Description
 
-Extension to ActiveRecord::Base for validating hostnames and domain names.
+Validate hostnames and domain names in Rails.
 
 
 == Features
@@ -16,6 +16,8 @@ Extension to ActiveRecord::Base for validating hostnames and domain names.
 
 == Installation
 
+Requires Rails 4 or newer.
+
 As plugin (from master)
 
   rails plugin install git://github.com/KimNorgaard/validates_hostname.git

Rakefile

@@ -14,20 +14,16 @@ spec = Gem::Specification.new do |s|
   s.version                   = GEM_VERSION
   s.required_rubygems_version = Gem::Requirement.new(">= 0") if s.respond_to? :required_rubygems_version=
   s.authors                   = ["Kim Nørgaard"]
-  s.description               = 'Extension to ActiveRecord::Base for validating hostnames'
+  s.description               = 'Extension to validate hostnames in Rails'
   s.summary                   = 'Checks for valid hostnames'
   s.email                     = '[email protected]'
   s.extra_rdoc_files          = ["README.rdoc", "CHANGELOG.rdoc", "MIT-LICENSE"]
   s.files                     = ["validates_hostname.gemspec", "MIT-LICENSE", "CHANGELOG.rdoc", "README.rdoc", "Rakefile", "lib/validates_hostname", "lib/validates_hostname/version.rb", "lib/validates_hostname.rb"]
   s.homepage                  = %q{https://github.com/KimNorgaard/validates_hostname}
   s.licenses                  = 'MIT'
   s.require_paths             = ["lib"]
-  s.add_runtime_dependency 'activerecord', '>= 3.0'
-  s.add_runtime_dependency 'activesupport', '>= 3.0'
+  s.add_runtime_dependency 'activemodel', '>= 4.0'
   s.add_development_dependency 'rspec', '~> 3.0'
-  s.add_development_dependency 'rspec-rails'
-  s.add_development_dependency 'rails'
-  s.add_development_dependency 'sqlite3'
   s.add_development_dependency 'pry-byebug'
   s.add_development_dependency 'rspec-collection_matchers'
 end