Update travis and phpunit (#489)

.travis.yml

@@ -1,7 +1,9 @@
 language: php
 
 php:
- - '7.0'
+  - 7.0
+  - 7.1
+  - 7.2
 
 services:
   - docker
@@ -11,7 +13,7 @@ cache:
     - $HOME/.composer/cache
 
 install:
- - composer install
+  - composer install --prefer-dist --no-interaction
   - composer run-script parse-stubs
 
 script:

composer.json

@@ -35,8 +35,7 @@
         "webmozart/path-util": "^2.3"
     },
     "require-dev": {
-        "phpunit/phpunit": "^5.5",
-        "phpunit/php-code-coverage": "^4.0"
+        "phpunit/phpunit": "^6.3"
     },
     "autoload": {
         "psr-4": {