Update coverage from 4.5.1 to 4.5.3

Author: pyup-bot

requirements_dev.txt

@@ -4,7 +4,7 @@ wheel==0.32.2
 watchdog==0.9.0
 flake8==3.6.0
 tox==3.5.3
-coverage==4.5.1
+coverage==4.5.3
 Sphinx==1.8.1
 twine==1.12.1
 sphinx_rtd_theme==0.4.2