Our style guide includes formatting conventions. Instead of wasting time in reviewing PRs discussing things like indenting, and to avoid later deltas to fix bad formatting in earlier commits, we want to enforce these in the build.
The intent in this change is to fail the build as quickly as possible in travis, so those sending a PR immediately know they should run clang-format and update their commit. When running locally, we want users to know about formatting, but they may not want to act on it immediately, until they are done working. For this reason, it is only a warning outside of the continuous build.
This is done by having a check-format task which should run on most local builds, and an enforce-format task only run by travis.
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