Adding Libretime PPA, removing Xenial, CentOS PHP tests

.travis.yml

@@ -7,6 +7,10 @@ env:
   global:
     - ENVIRONMENT=testing
     - LIBRETIME_LOG_DIR=/tmp/log/libretime
+    
+before_install:
+  - sudo add-apt-repository -y ppa:libretime/libretime
+  - sudo apt-get -q update
 
 addons:
   apt:
@@ -38,12 +42,6 @@ jobs:
     - language: php
       php: 7.2
       stage: test
-    - language: php
-      php: 7.0
-      stage: test
-    - language: php
-      php: 5.6
-      stage: test
     - language: python
       python: 3.8
       stage: test