Update PHP versions in CI

5.3 unsupported by Travis CI now
7.2 is RC
HVVM is 5.6 based and I think that -prefer-lowest is enough nowadays

.travis.yml

@@ -2,13 +2,15 @@ language: php
 
 matrix:
   include:
-    - php: 5.3
     - php: 5.4
     - php: 5.5
       env: COMPOSER_OPTS="--prefer-lowest"
     - php: 5.6
     - php: 7.0
+    - php: 7.1
+    - php: 7.2
     - php: hhvm
+      env: COMPOSER_OPTS="--prefer-lowest"
 
 before_install:
   - sudo apt-get update -qq