switched method of determining aws account id to STS

This is more stable, as it also works when using  temporary credentials.

lib/train/transports/aws.rb

@@ -62,9 +62,8 @@ def uri
 
       def unique_identifier
         # use aws account id
-        client = aws_client(::Aws::IAM::Client)
-        arn = client.get_user.user.arn
-        arn.split(':')[4]
+        client = aws_client(::Aws::STS::Client)
+        arn = client.get_caller_identity.account
       end
     end
   end

test/unit/transports/aws_test.rb

@@ -98,26 +98,23 @@ def initialize(hash)
   end
 
   describe 'unique_identifier' do
-    class AwsArn
-      def arn
-        'arn:aws:iam::158551926027:user/test-fixture-maker'
-      end
-    end
 
-    class AwsUser
-      def user
-        AwsArn.new
+    class AwsCallerId
+      def account
+        "123456789012"
       end
     end
 
-    class AwsClient
-      def get_user
-        AwsUser.new
+    class StsClient
+      def get_caller_identity
+        AwsCallerId.new
       end
     end
+
     it 'returns an account id' do
-      connection.stubs(:aws_client).returns(AwsClient.new)
-      connection.unique_identifier.must_equal '158551926027'
+      connection.stubs(:aws_client).returns(StsClient.new)
+      connection.unique_identifier.must_equal '123456789012'
     end
   end
+
 end