keep 'coverage' at a version older than 4.2
keep 'coverage' at a version older than 4.2

This is an attempt to fix TravisCI for builds on Python 3.2 by using an
older version of coverage.  See travis-ci/travis-ci#4866

JonathonReinhart/scuba,JonathonReinhart/scuba,JonathonReinhart/scuba