Make dist package smaller by removing tests and other non needed files for production

See https://blog.madewithlove.be/post/gitattributes/

.gitattributes

+/.gitattributes export-ignore
+/.gitignore export-ignore
+/.php_cs export-ignore
+/.scrutinizer.yml export-ignore
+/.travis.yml export-ignore
+/CHANGELOG.md export-ignore
+/CONTRIBUTING.md
+/examples export-ignore
+/phpunit.xml.dist export-ignore
+/README.md export-ignore
+/tests export-ignore