remove pytest-xdist, because of error within travis

.travis.yml

@@ -21,7 +21,7 @@ install:
   - conda create -q -n test-environment python=$TRAVIS_PYTHON_VERSION numpy
   - source activate test-environment
   - conda install -q -c https://conda.binstar.org/menpo opencv3
-  - pip install --upgrade pytest pytest-benchmark pytest-cov pytest-xdist pytest-flake8
+  - pip install --upgrade pytest pytest-benchmark pytest-cov pytest-flake8
   - pip install --upgrade coveralls sphinx_rtd_theme python-coveralls
   - pip install -r requirements.txt
   - pip install -e .