Run PG13 tests on CI (#44)

TopN approximation algorithm returns different and consistent results
on PG13 due to some changes in the execution of the plan. I added an
alternative test output to resolve this.

.travis.yml

@@ -12,6 +12,7 @@ matrix:
     - env: PGVERSION=10
     - env: PGVERSION=11
     - env: PGVERSION=12
+    - env: PGVERSION=13
 before_install:
   - bash test_data_provider
   - git clone -b v0.7.13 --depth 1 https://github.com/citusdata/tools.git

expected/customer_reviews_query.out

@@ -1,3 +1,10 @@
+SHOW server_version \gset
+SELECT substring(:'server_version', '\d+')::int >= 13;
+ ?column? 
+----------
+ t
+(1 row)
+
 -- Create table to insert summaries.
 create table popular_products
 (
@@ -40,9 +47,9 @@ order by 2 DESC, 1;
  0671014730 |       572
  0743527550 |       572
  0375700757 |       555
- 0871136791 |       555
+ 0871136791 |       553
  0679460691 |       551
- 0375402926 |       550
+ 0375402926 |       548
  0613221583 |       536
  0812550293 |       536
  1575110458 |       536

expected/customer_reviews_query_0.out

@@ -0,0 +1,99 @@
+SHOW server_version \gset
+SELECT substring(:'server_version', '\d+')::int >= 13;
+ ?column? 
+----------
+ f
+(1 row)
+
+-- Create table to insert summaries.
+create table popular_products
+(
+  review_summary jsonb,
+  year double precision,
+  month double precision
+);
+set topn.number_of_counters to 1000;
+-- Create different summaries by grouping the reviews according to their year and month.
+insert into
+  popular_products(review_summary, year, month )
+select
+  topn_add_agg(product_id),
+  extract(year from review_date) as year,
+  extract(month from review_date) as month
+from
+  customer_reviews
+group by
+  year,
+  month;
+-- Create another table for the merged results.
+create table
+  overall_result(merged_summary jsonb);
+-- Let's merge the summaries for the overall result and check top-20 items.
+insert into
+  overall_result(merged_summary)
+select
+  topn_union_agg(review_summary)
+from
+  popular_products;
+select
+  (topn(merged_summary, 20)).*
+from
+  overall_result
+order by 2 DESC, 1;
+    item    | frequency 
+------------+-----------
+ 0671003755 |       576
+ 0613033205 |       575
+ 0671014730 |       572
+ 0743527550 |       572
+ 0375700757 |       555
+ 0871136791 |       555
+ 0679460691 |       551
+ 0375402926 |       550
+ 0613221583 |       536
+ 0812550293 |       536
+ 1575110458 |       536
+ 1590073983 |       536
+ 1590073991 |       536
+ 0375402675 |       524
+ 0375403477 |       524
+ 0375703241 |       524
+ 0399143904 |       524
+ 0425170349 |       524
+ 0613222407 |       524
+ 0765342294 |       510
+(20 rows)
+
+-- Test window functions
+CREATE TABLE daily_populars
+(
+  date DATE,
+  agg_data JSONB
+);
+INSERT INTO daily_populars
+  SELECT
+    date_trunc('day', review_date),
+    topn_add_agg(product_id)
+  FROM 
+    customer_reviews
+  GROUP BY 
+    1;
+SELECT 
+  date, 
+  topn_union_agg(agg_data) OVER seven_days 
+FROM 
+  daily_populars 
+WINDOW 
+  seven_days AS (ORDER BY date ASC ROWS 6 PRECEDING)
+ORDER BY 
+  1
+LIMIT 5;
+    date    |                                                                               topn_union_agg                                                                               
+------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ 12-30-1970 | {"1551802538": 1, "1551803542": 1}
+ 06-19-1995 | {"0898624932": 1, "1551802538": 1, "1551803542": 1}
+ 06-23-1995 | {"0521469112": 1, "0898624932": 1, "1551802538": 1, "1551803542": 1}
+ 07-14-1995 | {"0521469112": 1, "0679722955": 1, "0898624932": 1, "1551802538": 1, "1551803542": 1}
+ 07-18-1995 | {"0195069056": 1, "0471114251": 1, "0517887290": 1, "0521469112": 1, "0679722955": 1, "0898624932": 1, "0962344788": 1, "1551802538": 1, "1551803542": 1, "1574531093": 1}
+(5 rows)
+

sql/customer_reviews_query.sql

@@ -1,3 +1,6 @@
+SHOW server_version \gset
+SELECT substring(:'server_version', '\d+')::int >= 13;
+
 -- Create table to insert summaries.
 create table popular_products
 (