[AIRFLOW-2553] Add webserver.pid to .gitignore

Closes #3451 from r39132/update_gitignore

.gitignore

@@ -14,6 +14,7 @@ unittests.db
 airflow/git_version
 airflow/www/static/coverage/
 logs/
+airflow-webserver.pid
 
 # Byte-compiled / optimized / DLL files
 __pycache__/