chore(dev_requirements): update coverage lib to 4.2.0

See https://coverage.readthedocs.io/en/coverage-4.2/changes.html#version-4-2-2016-07-26

Author: mboersma

rootfs/dev_requirements.txt

@@ -1,5 +1,5 @@
 # Run "make coverage" or "make test-unit" for the % of code exercised during tests
-coverage==4.1.0
+coverage==4.2.0
 
 # Run "make flake8" to check python syntax and style
 flake8==2.5.4