Fix compatibility with Ruby 1.9.3 and 2.0.0

The named arguments syntax used for the Rails 5 `digest` method is only
valid on Ruby 2.1 or higher (or 2.0.0 if default values are supplied),
causing an error parsing the lib/rabl/digestor.rb source on these
older versions.

    SyntaxError: rabl-0.13.0/lib/rabl/digestor.rb:6: syntax error, unexpected ','
          def self.digest(name:, finder:, **options)

The support for different Rails versions has been moved into separate
files that can be loaded only when required.

.travis.yml

@@ -5,7 +5,7 @@ rvm:
   - 1.9.3
   - 2.0.0
   - 2.1
-  - 2.2
+  - 2.2.5
   - 2.3.1
 notifications:
   recipients:

Gemfile

@@ -14,9 +14,10 @@ group :test do
   # RABL TEST
   gem 'builder'
 
+  rails_version = RUBY_VERSION =~ /\A(1|2.[01])/ ? '~> 4.0' : '>= 4.0'
   # FIXTURES
   gem 'rack-test', :require => 'rack/test'
-  gem 'activerecord', :require => 'active_record'
+  gem 'activerecord', rails_version, :require => 'active_record'
   gem 'sqlite3'
   gem 'sinatra', '>= 1.2.0'
   gem 'hashie'

lib/rabl/digestor.rb

@@ -3,45 +3,11 @@ class Digestor < ActionView::Digestor
     # Override the original digest function to ignore partial which
     # rabl doesn't use the Rails conventional _ symbol.
     if Gem::Version.new(Rails.version) >= Gem::Version.new('5.0.0.beta1')
-      def self.digest(name:, finder:, **options)
-
-        options.assert_valid_keys(:dependencies, :partial)
-
-        cache_key = ([ name ].compact + Array.wrap(options[:dependencies])).join('.')
-
-        # this is a correctly done double-checked locking idiom
-        # (Concurrent::Map's lookups have volatile semantics)
-        finder.digest_cache[cache_key] || @@digest_monitor.synchronize do
-          finder.digest_cache.fetch(cache_key) do # re-check under lock
-            begin
-              # Prevent re-entry or else recursive templates will blow the stack.
-              # There is no need to worry about other threads seeing the +false+ value,
-              # as they will then have to wait for this thread to let go of the @@digest_monitor lock.
-
-              pre_stored = finder.digest_cache.put_if_absent(cache_key, false).nil? # put_if_absent returns nil on insertion
-
-              finder.digest_cache[cache_key] = stored_digest = Digestor.new(name, finder, options).digest
-            ensure
-              # something went wrong or ActionView::Resolver.caching? is false, make sure not to corrupt the @@cache
-              finder.digest_cache.delete_pair(cache_key, false) if pre_stored && !stored_digest
-            end
-          end
-        end
-      end
+      require 'rabl/digestor/rails5'
     elsif Gem::Version.new(Rails.version) >= Gem::Version.new('4.1')
-      def self.digest(options = {})
-        cache_key = [options[:name]] + Array.wrap(options[:dependencies])
-        @@cache[cache_key.join('.')] ||= begin
-          Digestor.new({ :name => options[:name], :finder => options[:finder] }.merge!(options)).digest
-        end
-      end
+      require 'rabl/digestor/rails41'
     else
-      def self.digest(name, format, finder, options = {})
-        cache_key = [name, format] + Array.wrap(options[:dependencies])
-        @@cache[cache_key.join('.')] ||= begin
-          Digestor.new(name, format, finder, options).digest
-        end
-      end
+      require 'rabl/digestor/rails3'
     end
 
     private

lib/rabl/digestor/rails3.rb

@@ -0,0 +1,10 @@
+module Rabl
+  class Digestor < ActionView::Digestor
+    def self.digest(name, format, finder, options = {})
+      cache_key = [name, format] + Array.wrap(options[:dependencies])
+      @@cache[cache_key.join('.')] ||= begin
+        Digestor.new(name, format, finder, options).digest
+      end
+    end
+  end
+end

lib/rabl/digestor/rails41.rb

@@ -0,0 +1,10 @@
+module Rabl
+  class Digestor < ActionView::Digestor
+    def self.digest(options = {})
+      cache_key = [options[:name]] + Array.wrap(options[:dependencies])
+      @@cache[cache_key.join('.')] ||= begin
+        Digestor.new({ :name => options[:name], :finder => options[:finder] }.merge!(options)).digest
+      end
+    end
+  end
+end

lib/rabl/digestor/rails5.rb

@@ -0,0 +1,29 @@
+module Rabl
+  class Digestor < ActionView::Digestor
+    def self.digest(name:, finder:, **options)
+
+      options.assert_valid_keys(:dependencies, :partial)
+
+      cache_key = ([ name ].compact + Array.wrap(options[:dependencies])).join('.')
+
+      # this is a correctly done double-checked locking idiom
+      # (Concurrent::Map's lookups have volatile semantics)
+      finder.digest_cache[cache_key] || @@digest_monitor.synchronize do
+        finder.digest_cache.fetch(cache_key) do # re-check under lock
+          begin
+            # Prevent re-entry or else recursive templates will blow the stack.
+            # There is no need to worry about other threads seeing the +false+ value,
+            # as they will then have to wait for this thread to let go of the @@digest_monitor lock.
+
+            pre_stored = finder.digest_cache.put_if_absent(cache_key, false).nil? # put_if_absent returns nil on insertion
+
+            finder.digest_cache[cache_key] = stored_digest = Digestor.new(name, finder, options).digest
+          ensure
+            # something went wrong or ActionView::Resolver.caching? is false, make sure not to corrupt the @@cache
+            finder.digest_cache.delete_pair(cache_key, false) if pre_stored && !stored_digest
+          end
+        end
+      end
+    end
+  end
+end