build lowest dep versions on php56 and php70 to ensure that our ranges are okay. add hhvm build.

Author: abacaphiliac

.travis.yml

@@ -3,10 +3,20 @@ language: php
 php:
   - '5.6'
   - '7.0'
+  - 'hhvm'
+
+env:
+  - COMPOSER_OPTS=""
+  - COMPOSER_OPTS="--prefer-lowest"
+
+matrix:
+  allow_failures:
+    - php: hhvm
+  fast_finish: true
 
 before_script:
   - composer self-update
-  - composer install
+  - composer update
 
 script:
   - bin/phpunit --coverage-clover=coverage.clover