adjust build matrix and tell phpunit bridge we need 5.7

Author: dbu

.travis.yml

@@ -1,8 +1,6 @@
 language: php
 
 php:
-  - 5.5
-  - 5.6
   - 7.0
   - hhvm
 
@@ -18,10 +16,11 @@ env:
   global:
     - SYMFONY_DEPRECATIONS_HELPER=42
     - SYMFONY_PHPUNIT_DIR=.phpunit SYMFONY_PHPUNIT_REMOVE="symfony/yaml"
+    - SYMFONY_PHPUNIT_VERSION=5.7
 
 matrix:
   include:
-    - php: 5.5
+    - php: 5.6
       env: COMPOSER_FLAGS="--prefer-lowest" SYMFONY_VERSION=2.8.* SYMFONY_DEPRECATIONS_HELPER=weak
     - php: 7.1
       env: DEPS=dev SYMFONY_VERSION=3.3.*