Travis build fails because of corrupt Bazel installation

[adamcrume]

The build often fails with:

Uncompressing......Error: corrupt installation: file '/home/travis/.cache/bazel/_bazel_travis/install/eb8f56e396a53e354febafe5175189a2/_embedded_binaries/embedded_tools/third_party/py/gflags/init.py' missing. Please remove '/home/travis/.cache/bazel/_bazel_travis/install/eb8f56e396a53e354febafe5175189a2' and try again.

Clearing the cache and rebuilding fixes the error. Removing ~/.cache/bazel from the Travis cache should permanently fix the problem.