Drop pre 7.1

Drop pre 5.6 for constant arrays
Drop pre 7.1 for private constants

.travis.yml

@@ -29,20 +29,8 @@ jobs:
         - composer test-dead-code
 
 
-    - &UNIT_TEST
-      stage: Units
-      php: 5.5
-      install:
-        # remove packages with PHP requirements higher than 7.0 to prevent composer trying to resolve these, see: https://github.com/composer/composer/issues/6011
-        - composer remove vimeo/psalm friendsofphp/php-cs-fixer infection/infection --dev --no-update --no-interaction
-        - composer install --prefer-dist --no-interaction --no-progress
-      script: composer test-units
-    - <<: *UNIT_TEST
-      php: 5.6
-    - <<: *UNIT_TEST
-      php: 7.0
     - &MUTATION_AND_UNIT_TEST
-      <<: *UNIT_TEST
+      stage: Units
       php: 7.1
       install:
         - composer install --prefer-dist --no-interaction --no-progress

composer.json

@@ -13,7 +13,7 @@
         }
     ],
     "require": {
-        "php": "^7||^5.5",
+        "php": "^7.1",
         "ext-mbstring": "*"
     },
     "require-dev": {