Use composer-require-checker (#9)

Author: kubawerlos

Diff for: .composer-require-checker.json

@@ -0,0 +1,9 @@
+{
+    "symbol-whitelist": [
+        "PhpCsFixer\\PhpunitConstraintIsIdenticalString\\Constraint\\IsIdenticalString",
+
+        "null", "true", "false",
+        "static", "self", "parent",
+        "array", "string", "int", "float", "bool", "iterable", "callable", "void"
+    ]
+}

Diff for: .travis.yml

@@ -34,7 +34,7 @@ before_install:
     - composer global show -ND 2>&1 | grep "hirak/prestissimo" || travis_retry composer global require $DEFAULT_COMPOSER_FLAGS hirak/prestissimo
 
 install:
-    - composer update $DEFAULT_COMPOSER_FLAGS $COMPOSER_FLAGS
+    - travis_retry composer update $DEFAULT_COMPOSER_FLAGS $COMPOSER_FLAGS
     - composer info -D | sort
 
 script:
@@ -46,10 +46,12 @@ jobs:
             stage: Static code analysis
             php: 7.3
             install:
+                - travis_retry composer update $DEFAULT_COMPOSER_FLAGS
                 - travis_retry composer update -d dev-tools $DEFAULT_COMPOSER_FLAGS
                 - composer info -d dev-tools -D | sort
             script:
                 - composer validate --strict || travis_terminate 1
                 - composer normalize -d ./dev-tools ./../composer.json --dry-run || travis_terminate 1
+                - dev-tools/vendor/bin/composer-require-checker check composer.json --config-file=.composer-require-checker.json || travis_terminate 1
                 - dev-tools/vendor/bin/phpmd src,tests text phpmd.xml || travis_terminate 1
                 - dev-tools/vendor/bin/php-cs-fixer fix --diff --dry-run -v || travis_terminate 1