travis: remove the os.statvfs debug command

According to https://github.com/travis-ci/travis-ci/issues/2788 (which
was WONTFIXED), roughly half the travis-ci buildserver fleet runs off
tmpfs volumes, which have different notions of space-free and space-used
than real filesystems. We've adapted our tests to avoid relying upon
space-used>0, and no longer need this call to investigate the problem.

refs ticket:2290

.travis.yml

@@ -6,7 +6,6 @@ before_install:
   - sh -c set
   - git pull --depth=1000
   - python misc/build_helpers/show-tool-versions.py
-  - python -c "import os; print os.statvfs('.')"
 install:
   - pip install 'twisted>=0.13.0'
   - pip install 'coveralls==0.4.2'