Mark Postgres as an allowed failure

It breaks at the moment. Since we didn't test it before,
this isn't making the situation any worse, just more obvious.
See silverstripe#277

.travis.yml

@@ -19,6 +19,9 @@ matrix:
       env: NPM_TEST=1
     - php: 7.0
       env: DB=MYSQL PDO=1 PHPUNIT_COVERAGE_TEST=1
+  allow_failures:
+    - php: 5.5
+      env: DB=PGSQL PHPUNIT_TEST=1
 
 before_script:
     - composer self-update || true