[w3c/manifest] chore(.travis): check spec for errors/warnings (#739)

Part of #733 

This change (choose one):

* [x] Is a "chore" (metadata, formatting, fixing warnings, etc).

Commit message:

Check the spec on Travis, meaning we can drop the need to manually check for the spec. 
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/739

-- Commit Summary --

  * chore(.travis): check spec for errors/warnings

-- File Changes --

    M .travis.yml (3)
    A tools/validate.js (45)

-- Patch Links --

https://github.com/w3c/manifest/pull/739.patch
https://github.com/w3c/manifest/pull/739.diff

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