Travis: stop build testing against HHVM, test against nightly

At some point in time, PHP was lagging behind and HHVM seemed like the future. What with PHP 7 being released and doing incredibly well, that time has passed and the popularity of HHVM is now marginal.

This commit drops testing against HHVM and starts testing against bleeding edge PHP nightly instead.

.travis.yml

@@ -31,11 +31,11 @@ matrix:
         - php: '7.0'
         # aliased to a recent 7.2.x version
         - php: '7.2'
-        # aliased to a recent hhvm version
-        - php: 'hhvm'
+        # bleeding edge PHP
+        - php: 'nightly'
 
     allow_failures:
-        - php: 'hhvm'
+        - php: 'nightly'
 
 # Use this to prepare the system to install prerequisites or dependencies.
 # e.g. sudo apt-get update.