Fix the tox coverage target

This change prevents the following error:

$ tox -e coverage
...
coverage runtests: commands[1] | nosetests --with-coverage --cover-branches --cover-package honcho honcho/test/unit
...........................E......................
======================================================================
ERROR: Failure: ImportError (cannot import name 'coverage')
----------------------------------------------------------------------
Traceback (most recent call last):
  ...
  File "/Users/marca/dev/git-repos/honcho/honcho/test/unit/test_manager.py", line 10, in <module>
    multiprocessing.Process = monkeypatch_process_for_coverage(multiprocessing.Process)
  File "/Users/marca/dev/git-repos/honcho/honcho/test/helpers.py", line 87, in monkeypatch_process_for_coverage
    from coverage.control import coverage
ImportError: cannot import name 'coverage'

It seems that in coverage==4.0a1 (which is somehow in my company's local devpi server but not on PyPI now; wonder if @nedbat pushed it to PyPI and then had second thoughts?), "coverage" (lowercase 'c') changed to "Coverage" (uppercase 'c'). So it seems wise to pin to coverage 3.x for now.