don't run tests in parallel on travis

parallel tests might have been the cause of spurious travis failures

plus, not running concurrently should result in reusing the cache for the initial stages,
which might even save time

.travis.yml

@@ -6,10 +6,10 @@ python:
 - 3.4
 install:
 - pip install -e .
-- pip install pytest-xdist pyyaml
+- pip install pyyaml
 
 script:
-  - pytest -s -v -n4 tests/${REPO_TYPE}
+  - pytest -s -v tests/${REPO_TYPE}
 
 jobs:
   include: