Pull manlinux images that will be used to defeat travis-ci timeout

travis-ci seems to timeout with no output when pulling manylinux images.
pull them before install.

Author: mayeut

.travis.yml

@@ -31,6 +31,8 @@ install:
   - pip install -r test-requirements.txt
   - pip install tox codecov
   - pip install -e .
+  # pull manylinux images that will be used, this helps passing tests which would otherwise timeout.
+  - python -c $'from tests.integration.test_manylinux import MANYLINUX_IMAGES\nfor image in MANYLINUX_IMAGES.values():\n    print(image)' | xargs -L 1 docker pull
 
 script:
   - tests/travis.sh