Excluded Postgres from Travis (breaks build due to internal errors)

We'll need to fix the "no space left on device" issue,
most likely caused by Postgres keeping too much of a query log,
or somehow creating a history of past data.
For now, having a Postgres build breaking the whole
build process (incl. MySQL builds) does more harm than good.

.travis.yml

@@ -5,7 +5,6 @@ php:
 
 env:
  - TESTDB=MYSQL
- - TESTDB=PGSQL
  - TESTDB=SQLITE
 
 matrix: