Ensure coverage data persists accross test runs

.travis.yml

@@ -14,7 +14,7 @@ install:
   - pip install -U pytest pytest-cov pytest-xdist coveralls
   - pip install -e .
 script:
-  - py.test -n=auto --cov-config .coveragerc --cov=pyltc
+  - py.test -n=auto --cov-append --cov-config .coveragerc --cov=pyltc
   - py.test -n=1 --cov-append --cov-config .coveragerc --cov=pyltc --xdist-disabled tests/test_audio.py
 after_success:
   - coverage combine