List serializer no explicitly renders as 'not supported for HTML input'
Use the new build env on Travis

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Merge pull request #2090 from joshk/patch-1

Use the new build env on Travis
Allow blank choices to render. Closes #2071.
Merge branch 'master' of https://github.com/tomchristie/django-rest-framework
Use translatable error strings. Refs #2063.
Apply defaults and requiredness to unique_together fields. Closes #2092.
Further notes in 3.0 announcement
Merge branch 'master' into 3.0-beta