chore(dev_requirements): update coverage lib to 4.4.1

See https://coverage.readthedocs.io/en/coverage-4.4.1/changes.html#version-4-4-1-2017-05-14

Author: mboersma

rootfs/dev_requirements.txt

@@ -1,5 +1,5 @@
 # Run "make test-unit" for the % of code exercised during tests
-coverage==4.3.4
+coverage==4.4.1
 
 # Run "make test-style" to check python syntax and style
 flake8==3.3.0