diff --git a/.gitattributes b/.gitattributes
new file mode 100644
index 0000000..cf62a55
--- /dev/null
+++ b/.gitattributes
@@ -0,0 +1,7 @@
+/tests export-ignore
+/.gitattributes export-ignore
+/.gitignore export-ignore
+/.scrutinizer.yml export-ignore
+/.travis.yml export-ignore
+/phpunit.xml.dist export-ignore
+/README.md export-ignore
diff --git a/.travis.yml b/.travis.yml
index 70e479a..5905844 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -10,10 +10,16 @@ php:
   - 7.0
   - 7.1
 
+cache:
+  directories:
+    - $HOME/.composer/cache
+
 before_script:
-  - composer install --dev
+  - travis_retry composer install --dev
 
-script: phpunit --coverage-clover=coverage.clover
+script:
+  - composer validate --strict
+  - vendor/bin/phpunit --coverage-clover=coverage.clover
 
 after_script:
   - wget https://scrutinizer-ci.com/ocular.phar
diff --git a/composer.json b/composer.json
index 77b016d..73e4612 100644
--- a/composer.json
+++ b/composer.json
@@ -18,7 +18,8 @@
         "php": ">=5.3.0"
     },
     "require-dev": {
-        "ext-soap": "*"
+        "ext-soap": "*",
+        "phpunit/phpunit": "^4.0 || ^5.0"
     },
     "suggest": {
         "ext-soap": "Required if you want to check the VAT number via VIES"