MINOR: Drop dual MYSQL/PGSQL builds for PHP7

We can get away with testing MySQL on PHP 5.6
and PHP 7.1 and testing PGSQL on PHP 7.0. We don’t
need a full matrix.

.travis.yml

@@ -16,10 +16,6 @@ env:
 matrix:
   fast_finish: true
   include:
-    - php: 7.0
-      env:
-        - DB=PGSQL
-        - PHPUNIT_TEST=framework
     - php: 5.6
       env:
         - DB=MYSQL
@@ -27,7 +23,7 @@ matrix:
         - PHPCS_TEST=1
     - php: 7.0
       env:
-        - DB=MYSQL
+        - DB=PGSQL
         - PHPUNIT_TEST=framework
     - php: 7.1.2
       env: