Drop HHVM support

Because HHVM usage is very low (https://seld.be/notes/php-versions-stats-2017-1-edition)
and because we don't have the necessary resources to properly support it, we drop all
support for HHVM

.travis.yml

@@ -6,14 +6,11 @@ php:
   - 5.6
   - 7.0
   - 7.1
-  - hhvm
 
 matrix:
   include:
     - php: 7.0
       env: COVERAGE=1
-  allow_failures:
-    - php: hhvm
 
 cache:
   directories:

CHANGELOG.md

@@ -40,6 +40,7 @@ For a comprehensive list of all class changes, and a semi-automated migration pa
 
 - Dropped `PHPExcel_Calculation_Functions::VERSION()`. Composer or git should be used to know the version.
 - Dropped `PHPExcel_Settings::setPdfRenderer()` and `PHPExcel_Settings::setPdfRenderer()`. Composer should be used to autoload PDF libs.
+- Dropped support for HHVM
 
 ## Previous versions of PHPExcel