Enable Travis cache for pip, npm and bower
Enable Travis cache for pip, npm and bower

We've seen travis builds fail sometimes during npm install, so
maybe [caching the dependency paths][1] will help to avoid this.

This might introduce other issues though, with stale cache causing
the build to run outdated code, so keep in mind you might need to
clear the travis cache if the build fails unexpectedly.

[1]: https://docs.travis-ci.com/user/caching

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