Add gittattributes for lower download size

Reference: https://www.reddit.com/r/PHP/comments/2jzp6k/i_dont_need_your_tests_in_my_production/

Author: vinkla

.gitattributes

@@ -0,0 +1,11 @@
+/tests export-ignore
+/.gitattributes export-ignore
+/.gitignore export-ignore
+/.travis.yml export-ignore
+/.scrutinizer.yml export-ignore
+/.php_cs export-ignore
+/phpunit.xml.dist export-ignore
+/phpspec.xml.dist export-ignore
+/CHANGELOG.md export-ignore
+/CONTRIBUTING.md export-ignore
+/README.md export-ignore