[manifest] Don't publish if it's a pull request (#429)

Without this, a publication gets triggered even if it's a pull request.

You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/429

-- Commit Summary --

  * Don't publish if it's a pull request

-- File Changes --

    M .travis.yml (4)

-- Patch Links --

https://github.com/w3c/manifest/pull/429.patch
https://github.com/w3c/manifest/pull/429.diff

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/manifest/pull/429