minor #25 DX: Add PHP 7.3 to Travis and upgrade PHP CS Fixer (keradus)

This PR was squashed before being merged into the master branch (closes #25).

Discussion
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DX: Add PHP 7.3 to Travis and upgrade PHP CS Fixer

Replaces #8

Commits
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5a93879 DX: Travis - add job running on PHP 7.3
08042a5 DX: Upgrade PHP CS Fixer to one that supports PHP 7.3

Author: weaverryan

.travis.yml

@@ -18,6 +18,7 @@ matrix:
           env: deps=high
         - php: 7.2
           env: deps=low
+        - php: 7.3
 
     allow_failures:
           # Dev-master is allowed to fail.

composer.json

@@ -28,7 +28,7 @@
         "symfony/contracts": "^1.0"
     },
     "require-dev": {
-        "friendsofphp/php-cs-fixer": "^2.13",
+        "friendsofphp/php-cs-fixer": "^2.14",
         "symfony/framework-bundle": "^3.4 || ^4.0",
         "symfony/phpunit-bridge": "^3.4 || ^4.1",
         "symfony/twig-bundle": "^3.4 || ^4.0",