Remove PHPDocumentor & change travis platform for hhvm tests (jsonrainbow#429)

* Remove dev-time dependency on phpdocumentor due to resolution headaches

* Switch distro for hhvm testing to trusty (precise now unsupported)

Author: erayd

.travis.yml

@@ -18,6 +18,7 @@ matrix:
     - php: 7.1
     - php: 'nightly'
     - php: hhvm
+      dist: trusty
   allow_failures:
     - php: 'nightly'
 

composer.json

@@ -40,9 +40,8 @@
     },
     "require-dev": {
         "json-schema/JSON-Schema-Test-Suite": "1.2.0",
-        "phpunit/phpunit": "^4.8.22",
         "friendsofphp/php-cs-fixer": "^2.1",
-        "phpdocumentor/phpdocumentor": "^2.7"
+        "phpunit/phpunit": "^4.8.22"
     },
     "autoload": {
         "psr-4": { "JsonSchema\\": "src/JsonSchema/" }