Auto merge of #23184 - servo:jdm-patch-28, r=SimonSapin
Auto merge of #23184 - servo:jdm-patch-28, r=SimonSapin

Force python 2.7 on TravisCI.

From TravisCI:
```
 This job is running using the default Python version, which will be updated to 3.6 on April 16th, 2019. You can explicitly stay on the previous version by specifying python: 2.7 in your .travis.yml.
```

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