Update selenium from 3.12.0 to 3.13.0 (#104)

Author: pyup-bot

axe_selenium_python/tests/requirements/tests.txt

@@ -1,5 +1,5 @@
 pytest==3.6.2
-selenium==3.12.0
+selenium==3.13.0
 pytest-selenium==1.13.0
 pytest-html==1.19.0
 pytest_base_url==1.4.1