Travis: change from "trusty" to "xenial"

As the "trusty" environment is no longer officially supported by Travis, they decided in their wisdom to silently stop updating the PHP "nightly" image, which makes it next to useless as the last image apparently is from January....

This updates the Travis config to:
* Remove the global `dist` key, which let's the build use the system default, which at this time is `xenial`.
* Sets the distro for low PHP versions explicitly to `trusty`.
* Makes the expected OS explicit (linux).
* Remove redundant `stage` keys.

Author: jrfnl

.travis.yml

@@ -1,5 +1,4 @@
-dist: trusty
-
+os: linux
 language: php
 
 ## Cache composer and apt downloads.
@@ -11,10 +10,8 @@ cache:
     # Cache directory for more recent Composer versions.
     - $HOME/.cache/composer/files
 
-# Note: PHP 7.3, 7.4 and nightly are added via "jobs".
+# Note: PHP 5.4, 5.5, 7.3, 7.4 and nightly are added via "jobs".
 php:
-  - 5.4
-  - 5.5
   - 5.6
   - 7.0
   - 7.1
@@ -70,15 +67,14 @@ jobs:
     - stage: quicktest
       php: 7.4
       env: PHPCS_VERSION="dev-master" LINT=1
-    - stage: quicktest
-      php: 7.2
+    - php: 7.2
       env: PHPCS_VERSION="3.1.0"
 
-    - stage: quicktest
-      php: 5.4
+    - php: 5.4
+      dist: trusty
       env: PHPCS_VERSION="dev-master" LINT=1
-    - stage: quicktest
-      php: 5.4
+    - php: 5.4
+      dist: trusty
       env: PHPCS_VERSION="3.1.0"
 
     #### TEST STAGE ####
@@ -95,9 +91,22 @@ jobs:
     - php: 7.4
       env: PHPCS_VERSION="3.5.0"
 
+    # PHP 5.4/5.5 need trusty.
+    - php: 5.5
+      dist: trusty
+      env: PHPCS_VERSION="dev-master" LINT=1
+    - php: 5.5
+      dist: trusty
+      env: PHPCS_VERSION="3.1.0"
+    - php: 5.4
+      dist: trusty
+      env: PHPCS_VERSION="dev-master" LINT=1
+    - php: 5.4
+      dist: trusty
+      env: PHPCS_VERSION="3.1.0"
+
     # Builds against unstable versions which are allowed to fail for now.
-    - stage: test
-      php: 7.4
+    - php: 7.4
       env: PHPCS_VERSION="4.0.x-dev"
     - php: "nightly"
       env: PHPCS_VERSION="dev-master" LINT=1