Don't allow PG11 travis failures anymore

We made PG11 builds optional when we had an issue
with mx isolation test that we could not solve back then.

This commit solves the issue with a workaround by running
start_metadata_sync_to_node outside the transaction block.

.travis.yml

@@ -21,8 +21,6 @@ matrix:
     - env: PGVERSION=9.6
     - env: PGVERSION=10
     - env: PGVERSION=11
-  allow_failures:	
-    - env: PGVERSION=11
 before_install:
   - git clone -b v0.7.9 --depth 1 https://github.com/citusdata/tools.git
   - sudo make -C tools install

src/test/regress/specs/isolation_get_distributed_wait_queries.spec

@@ -19,9 +19,17 @@ setup
 
 	SELECT citus.replace_isolation_tester_func();
 	SELECT citus.refresh_isolation_tester_prepared_statement();
-
+	
-	SELECT start_metadata_sync_to_node('localhost', 57637);
+	-- start_metadata_sync_to_node can not be run inside a transaction block
-	SELECT start_metadata_sync_to_node('localhost', 57638);
+	-- following is a workaround to overcome that
+	-- port numbers are hard coded at the moment
+	SELECT master_run_on_worker(
+		ARRAY['localhost']::text[],
+		ARRAY[57636]::int[],
+		ARRAY[format('SELECT start_metadata_sync_to_node(''%s'', %s)', nodename, nodeport)]::text[],
+		false)
+	FROM pg_dist_node;
+	
 	SET citus.shard_replication_factor TO 1;
 	SET citus.replication_model to streaming;
 

src/test/regress/specs/isolation_reference_on_mx.spec

@@ -20,8 +20,16 @@ setup
   	SELECT citus.replace_isolation_tester_func();
   	SELECT citus.refresh_isolation_tester_prepared_statement();
 
-	SELECT start_metadata_sync_to_node('localhost', 57637);
+	-- start_metadata_sync_to_node can not be run inside a transaction block
-	SELECT start_metadata_sync_to_node('localhost', 57638);
+	-- following is a workaround to overcome that
+	-- port numbers are hard coded at the moment
+	SELECT master_run_on_worker(
+		ARRAY['localhost']::text[],
+		ARRAY[57636]::int[],
+		ARRAY[format('SELECT start_metadata_sync_to_node(''%s'', %s)', nodename, nodeport)]::text[],
+		false)
+	FROM pg_dist_node;
+
 	SET citus.replication_model to streaming;
 
 	CREATE TABLE ref_table(user_id int, value_1 int);