fixup! Support coverage for subprocess. (#399)

Add missing egg `coverage` to part that imports coverage on
initialization.

buildout.cfg

@@ -23,6 +23,7 @@ recipe = zc.recipe.testrunner
 eggs =
   zc.buildout[test]
   zc.recipe.egg
+  coverage
 initialization =
   import os
   if 'COVERAGE_PROCESS_START' not in os.environ: os.environ['COVERAGE_PROCESS_START'] = os.path.abspath('../../.coveragerc')