Unit tests did not catch breaking change from dependency update.

[b4handjr]

A breaking change was introduced here but it seems the unit tests run on travisci did not catch the build failure. The integration-tests run on circleci for both addons-frontend and addons-server have been failing due to this update.

We should investigate why the unit tests did not catch the build failure. A PR to pin the version to a known working version has been submitted.

[kumar303]

It was a webpack failure, see #11544

The unit tests should definitely catch this. I haven't looked into why they didn't but since we did not get a failing greenkeeper PR for the postcss-loader update, I suspect they didn't catch it.

[willdurand]

The unit tests should definitely catch this. I haven't looked into why they didn't but since we did not get a failing greenkeeper PR for the postcss-loader update, I suspect they didn't catch it.

We did not get any Greenkeeper PR actually, I think that's the main reason. Otherwise, we would have caught this error, especially given the error logged in #11544.

[willdurand]

Based on this PR/branch: mozilla/addons-frontend#5884, which is all green, I pushed a new commit on top of it, with an invalid webpack config in the postcss-loader options: mozilla/addons-frontend@ccde17e.

The build fails as expected: https://travis-ci.org/mozilla/addons-frontend/builds/414016918

I don't think there is an issue with Travis CI not catching an issue in the build process. I think the issue is that we did not get a PR on time from Greenkeeper. I believe that this is a wontfix and we should spend time on #10714.