update .travis.yml to allow for failures on hhvm and nightly builds

Author: ifsnop

.travis.yml

@@ -5,6 +5,7 @@ group: deprecated-2017Q4
 language: php
 
 php:
+  - 7.2
   - 7.1
   - 7.0
   - 5.6
@@ -14,6 +15,11 @@ php:
   - hhvm
   - nightly
 
+matrix:
+  allow_failures:
+  - php: nightly
+  - php: hhvm
+
 services:
   - mysql