build: Make Travis CI 'Postgres' build non-voting

This will make it easier to detect regressions by not making all
builds marked as fail, but only if one of the other three fails.

Mute the known failure from Postgres by adding it to an allow_failures
section. It'll still run every commit, but is non-voting.
To be re-enabled once T75174 is fixed.

Bug: T75176
Change-Id: I2ea415edd308f2a012ef240d562c0073f15b9118

.travis.yml

@@ -31,6 +31,11 @@ matrix:
       php: hhvm-3.12
     - env: dbtype=mysql dbuser=root
       php: 7
+  allow_failures:
+    # Postgres support for unit tests is still buggy
+    # https://phabricator.wikimedia.org/T75174
+    - env: dbtype=postgres dbuser=travis
+      php: 5.5
 
 services:
   - mysql