Add tests for PHP 7.2 and 7.3

Allow those to fail, as we expect them to until we upgrade PHPUnit to one compatible with 7.2+.

Also allow HHVM to fail, because we're going to be deprecating it soon anyway.

Closes chrisboulton/php-resque#358

.travis.yml

@@ -1,13 +1,19 @@
 language: php
 php:
-  - 5.6
-  - 7.0
-  - 7.1
+  - '7.3'
+  - '7.2'
+  - '7.1'
+  - '7.0'
+  - '5.6'
   - hhvm
 matrix:
   exclude:
     - php: hhvm
       env: ENABLE_REDIS_EXT=1
+  allow_failures:
+    - php: '7.3'
+    - php: '7.2'
+    - php: hhvm
 env:
   - ENABLE_REDIS_EXT=0
   - ENABLE_REDIS_EXT=1