reduce chunk size for countries table (#1279)

One user reported the load of the "countries" table exceed
max_allowed_packet which in some configurations can be as low as 1MB.

Changing the chunk size from 500 to 50 has a small cost on the initial
load of the data (one additional second to the 17 taken previously)
while being more universally usable without changing the configuration
of the mysql server.
The new packet size is estimated to be about 500KB.

The committer has not checked other tables

caravel/data/__init__.py

@@ -178,7 +178,7 @@ def load_world_bank_health_n_pop():
         tbl_name,
         db.engine,
         if_exists='replace',
-        chunksize=500,
+        chunksize=50,
         dtype={
             'year': DateTime(),
             'country_code': String(3),