[STORE] Use text instead of string for the <String> data types

In order to be compatible with Elasticsearch 5.x, the default
property type for Ruby <String> objects has been changed to
`text` from `string`.

See: https://www.elastic.co/guide/en/elasticsearch/reference/current/string.html

Closes elastic#649

Author: jalberto

elasticsearch-persistence/lib/elasticsearch/persistence/model/base.rb

@@ -70,7 +70,7 @@ module Utils
         def lookup_type(type)
           case
             when type == String
-              'string'
+              'text'
             when type == Integer
               'integer'
             when type == Float

elasticsearch-persistence/test/unit/model_gateway_test.rb

@@ -70,7 +70,7 @@ def run!; DummyGatewayModel.gateway { |g| @b += 1 }; end
     end
 
     should "properly look up types for classes" do
-      assert_equal 'string',  Elasticsearch::Persistence::Model::Utils::lookup_type(String)
+      assert_equal 'text',    Elasticsearch::Persistence::Model::Utils::lookup_type(String)
       assert_equal 'integer', Elasticsearch::Persistence::Model::Utils::lookup_type(Integer)
       assert_equal 'float',   Elasticsearch::Persistence::Model::Utils::lookup_type(Float)
       assert_equal 'date',    Elasticsearch::Persistence::Model::Utils::lookup_type(Date)