Update PHP versions in CI

5.3 supported by Travis CI now in precise only
7.2 is RC
7.3 is nightly
HVVM is 5.6 based and I think that -prefer-lowest is enough nowadays

.travis.yml

@@ -3,12 +3,19 @@ language: php
 matrix:
   include:
     - php: 5.3
+      dist: precise
     - php: 5.4
     - php: 5.5
       env: COMPOSER_OPTS="--prefer-lowest"
     - php: 5.6
     - php: 7.0
+    - php: 7.1
+    - php: 7.2
+    - php: nightly
     - php: hhvm
+      env: COMPOSER_OPTS="--prefer-lowest"
+  allow_failures:
+    - php: nightly
 
 before_install:
   - sudo apt-get update -qq
@@ -19,4 +26,4 @@ install:
   - composer update $COMPOSER_OPTS
 
 script:
-  - phpunit tests
+  - vendor/bin/phpunit