Add PHP 7.1 and 7.2 to .travis.yml.

Use composer install instead of composer update.

Author: tomzx

.travis.yml

@@ -4,13 +4,15 @@ php:
     - 5.5
     - 5.6
     - 7.0
+    - 7.1
+    - 7.2
     - hhvm
 
 sudo: false
 
 before_script:
     - composer self-update
-    - composer update
+    - composer install --no-interaction --prefer-dist
 
 script:
     - php vendor/bin/phpunit