Drop HHVM and use the build matrix

HHVM LTS 3.24 is the last version to support PHP 5 compatibility
third paragraph states "HHVM will not aim to target PHP7" and further down "We do not intend to be the runtime of choice for folks with pure PHP7 code." (just HACK)
https://hhvm.com/blog/2017/09/18/the-future-of-hhvm.html

Use build matrix
uses sudo false for faster container based testing when sudo is not used

.travis.yml

@@ -1,12 +1,13 @@
 language: php
-php:
-  - 7.1
-  - 7.2
-  - hhvm
-  - nightly
+sudo: false
 
 matrix:
-    allow_failures:
-      - php: hhvm
-      - php: nightly
-script: "composer install && phpunit tests && php example.php"
+  fast_finish: true
+  include:
+    - php: 7.1
+    - php: 7.2
+    - php: nightly
+  allow_failures:
+    - php: nightly
+
+script: "composer install && phpunit tests && php example.php"