Default to proper string attributes if using Sphinx 2.0.x.

Filtering doesn't work, but it didn't with the str2ordinal attributes either - sorting works fine.

lib/thinking_sphinx/attribute.rb

@@ -11,6 +11,21 @@ module ThinkingSphinx
   class Attribute < ThinkingSphinx::Property
     attr_accessor :query_source
 
+    SphinxTypeMappings = {
+      :multi     => :sql_attr_multi,
+      :datetime  => :sql_attr_timestamp,
+      :string    => :sql_attr_str2ordinal,
+      :float     => :sql_attr_float,
+      :boolean   => :sql_attr_bool,
+      :integer   => :sql_attr_uint,
+      :bigint    => :sql_attr_bigint,
+      :wordcount => :sql_attr_str2wordcount
+    }
+
+    if Riddle.loaded_version.to_i > 1
+      SphinxTypeMappings[:string] = :sql_attr_string
+    end
+
     # To create a new attribute, you'll need to pass in either a single Column
     # or an array of them, and some (optional) options.
     #
@@ -117,16 +132,7 @@ def to_select_sql
     end
 
     def type_to_config
-      {
-        :multi     => :sql_attr_multi,
-        :datetime  => :sql_attr_timestamp,
-        :string    => :sql_attr_str2ordinal,
-        :float     => :sql_attr_float,
-        :boolean   => :sql_attr_bool,
-        :integer   => :sql_attr_uint,
-        :bigint    => :sql_attr_bigint,
-        :wordcount => :sql_attr_str2wordcount
-      }[type]
+      SphinxTypeMappings[type]
     end
 
     def include_as_association?