Update .gitignore to test workaround for RTFD

Add _build to .gitignore as per readthedocs/readthedocs.org#665

Author: ivanlanin

.gitignore

@@ -5,6 +5,7 @@
 Thumbs.db
 Desktop.ini
 .idea
+_build
 phpunit.xml
 composer.lock
 composer.phar
@@ -17,4 +18,4 @@ vendor
 *.docx
 *.rtf
 *.txt
-*.xml
+*.xml