build: Change Travis postgres user "root" back to "travis"

Follows-up 5168cb6, in which we moved from Precise vms to Trusty vms.
As a side-effect, the undocumented behaviour of the mysql user "travis"
having create-db rights was no longer. As such, we changed it to "root",
per <https://docs.travis-ci.com/user/database-setup/>.

However, this broke Postgres builds since those should still use
"travis". There is no user named "root" for postgres.

* Add 'dbuser' to the matrix environment.
* Improve inline documentation.

Bug: T75176
Change-Id: I09fc0a1da8737e71b3d2b4b88d72b58c150519c4

Author: Krinkle

.travis.yml

@@ -7,23 +7,30 @@
 # complement that setup by testing MediaWiki on travis
 #
 language: php
-# Using HHVM-3.6+ requires Trusty (Travis default: precise)
-# https://docs.travis-ci.com/user/languages/php#HHVM-versions
-# https://github.com/travis-ci/travis-ci/issues/7368
+# Use the slower sudo-enabled VMs instead of fast containers:
+# - Package 'djvulibre-bin' is not yet whitelisted for trusty containers.
+#   https://github.com/travis-ci/apt-package-whitelist/issues/4036
 sudo: required
 group: edge
+# Use Trusty instead of Travis default (precise)
+# - Required in order to use HHVM 3.6 or higher.
+# - Required for non-buggy xml library for XmlTypeCheck/UploadBaseTest (T75176).
 dist: trusty
 
 matrix:
   fast_finish: true
   include:
-    - env: dbtype=mysql
+    # On Trusty, mysql user 'travis' doesn't have create database rights
+    # Postgres has no user called 'root'.
+    - env: dbtype=mysql dbuser=root
       php: 5.5
-    - env: dbtype=postgres
+    - env: dbtype=postgres dbuser=travis
       php: 5.5
-    - env: dbtype=mysql
+    - env: dbtype=mysql dbuser=root
+      # https://docs.travis-ci.com/user/languages/php#HHVM-versions
+      # https://github.com/travis-ci/travis-ci/issues/7368
       php: hhvm-3.12
-    - env: dbtype=mysql
+    - env: dbtype=mysql dbuser=root
       php: 7
 
 services:
@@ -32,7 +39,7 @@ services:
 branches:
   # Test changes in master and arbitrary Travis CI branches only.
   # The latter allows developers to enable Travis CI in their GitHub fork of
-  # wikimedia/mediawiki and then push changes they like to test to branches like
+  # wikimedia/mediawiki and then push changes for testing to branches like
   # "travis-ci/test-this-awesome-change".
   only:
     - master
@@ -50,7 +57,7 @@ before_script:
       --pass travis
       --dbtype "$dbtype"
       --dbname traviswiki
-      --dbuser root
+      --dbuser "$dbuser"
       --dbpass ""
       --scriptpath "/w"