Auto merge of #12386 - servo:geckolib-travis, r=nox
Auto merge of #12386 - servo:geckolib-travis, r=nox

Build geckolib on travis.

This adds about two minutes to the twenty-minute build.

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This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/12386)
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