Use bazel disk cache with travis

[simon-mo]

Address #4982 for travis setting.

This saves all build result in $HOME/ray-bazel-cache and ask travis to cache the directory.

• https://docs.travis-ci.com/user/caching/
• https://docs.bazel.build/versions/master/remote-caching.html#disk-cache

[AmplabJenkins]

Test FAILed.
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Test FAILed.

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[pcmoritz]

This looks great! I'm rerunning the Travis build to see if the cache is actually used.

EDIT: Looks like it shaves ~30 to 40 mins from each of the first 4 builds, great job!