Upgrade Travis config to trusty, remove PHP 5.3 build

Can't adopt Xenial by default as it doesn't include earlier PHP versions

.travis.yml

@@ -2,26 +2,22 @@
 
 sudo: false
 
-dist: precise
+dist: trusty
 
 language: php
 
-php:
-  - 5.3
-  - 5.4
-  - 5.5
-
 env:
   - DB=MYSQL CORE_RELEASE=3.6
 
 matrix:
   include:
+    - php: 5.4
+    - php: 5.5
     - php: 5.6
       env: DB=PGSQL CORE_RELEASE=3
     - php: 5.6
       env: DB=PGSQL CORE_RELEASE=3.6
     - php: 7.1
-      env: DB=MYSQL CORE_RELEASE=3.6
 
 before_script:
   - composer self-update || true