Attempt to make tests more reliable: travis should only install postgres once

Update .travis.yml (parse-community#6490)

* Update .travis.yml

testing error to see what happens...

* Update .travis.yml

Attempting to resolve postgres in CL by installing postgis via sudo instead of through apt/packages

* Update .travis.yml

* Update .travis.yml

* Update .travis.yml

Removed extra lines of postgres that were under "services" and "addons". I believe the "postgresql" line under "services" was installing the default of 9.6 and "addons" was installing postgres 11. My guess is the fail was occurring due to 9.6 being called sometimes and it never had postgis installed. If this is true, the solution is to only install one version of postgres, which is version 11 with postgis 2.5.

Author: cbaker6

.travis.yml

@@ -2,14 +2,12 @@ language: node_js
 os: linux
 dist: xenial
 services:
-- postgresql
 - redis
 - docker
 addons:
   postgresql: '11'
   apt:
     packages:
-    - postgresql-11
     - postgresql-11-postgis-2.5
     - postgresql-11-postgis-2.5-scripts
 branches: