Drop HHVM support

Not dropping any of the HHVM workaround code yet. If someone sorts
out the Travis build, I'm fine with keeping HHVM support.

.travis.yml

@@ -10,18 +10,14 @@ php:
   - 7.0
   - 7.1
   - nightly
-  - hhvm
 
 install:
   - if [ $TRAVIS_PHP_VERSION = '7.0' ]; then composer require satooshi/php-coveralls '~1.0'; fi
   - composer install --prefer-dist
-  # Disable JIT compilation in hhvm, as the JIT is useless for short live scripts like tests.
-  - if [[ $TRAVIS_PHP_VERSION = hhvm* ]]; then echo -e "hhvm.jit = 0\nhhvm.php7.all = 1" >> /etc/hhvm/php.ini; fi
 
 matrix:
   allow_failures:
     - php: nightly
-    - php: hhvm
   fast_finish: true
 
 script:

UPGRADE-4.0.md

@@ -6,6 +6,8 @@ Upgrading from PHP-Parser 3.x to 4.0
 PHP-Parser now requires PHP 7.0 or newer to run. It is however still possible to *parse* PHP 5.2-5.6
 source code, while running on a newer version.
 
+Because HHVM does not support PHP 7, HHVM is no longer supported.
+
 ### Changes to the node structure
 
 * Many subnodes that previously held simple strings now store `Identifier` nodes instead (or