coverage: don't use py.test --boxed when using --cov

Seems that if we run --cov together with --boxed we get very low
coverage reports.  Not sure why they are underreading.  But we
introduced --boxed for pypy and will use it for cPO and fPO.  So for now
don't run coverage on these segfaulting platforms.

.travis.yml

@@ -31,7 +31,8 @@ script:
   # Compile all the files
   - python -m compileall -q -f .
   - python setup.py --quiet build
-  - py.test --boxed --cov=. -r EfsxX
+  - if [[ $TRAVIS_PYTHON_VERSION != 'pypy' ]]; then py.test --cov=. -r EfsxX; fi
+  - if [[ $TRAVIS_PYTHON_VERSION == 'pypy' ]]; then py.test --boxed -r EfsxX; fi
   - ./tools/pep8.sh travis
   # -W report warnings as errors when building docs
   - cd docs; make SPHINXOPTS=-W html; cd ..