Tests on php 7.3 (#575)

* Tests on php 7.3
* bumping php-cs-fixer version as per comment from @erayd on justinrainbow/json-schema#563
* turning yoda style off to preserve previous behaviour
* some EOL versions of php require an older version of php-cs-fixer
* attempting to normalise behaviour with php-cs-fixer 2.2
* attempt separate run of php 7.0 to avoid having xdebug loaded when running php-cs-fixer

.php_cs.dist

@@ -21,8 +21,11 @@ $config
         'phpdoc_order' => true,
         'phpdoc_summary' => false,
         'pre_increment' => false,
+        'increment_style' => false,
         'simplified_null_return' => false,
         'trailing_comma_in_multiline_array' => false,
+        'yoda_style' => false,
+        'phpdoc_types_order' => array('null_adjustment' => 'none', 'sort_algorithm' => 'none'),
     ))
     ->setFinder($finder)
 ;

.travis.yml

@@ -15,9 +15,12 @@ matrix:
     - php: 5.5
     - php: 5.6
     - php: 7.0
-      env: WITH_COVERAGE=true WITH_PHPCSFIXER=true
+      env: WITH_COVERAGE=true
+    - php: 7.0
+      env: WITH_PHPCSFIXER=true
     - php: 7.1
     - php: 7.2
+    - php: 7.3
     - php: 'nightly'
     - php: hhvm
       dist: trusty

composer.json

@@ -30,7 +30,7 @@
         "php": ">=5.3.3"
     },
     "require-dev": {
-        "friendsofphp/php-cs-fixer": "~2.2.20",
+        "friendsofphp/php-cs-fixer": "~2.2.20||~2.15.1",
         "json-schema/JSON-Schema-Test-Suite": "1.2.0",
         "phpunit/phpunit": "^4.8.35"
     },