[manifest] Use `https://` where possible (#355)

Don't forget to also change the link from the description of the repository:

![](https://cloud.githubusercontent.com/assets/1223565/7035084/1c921a94-dd8e-11e4-8bb3-8667fc257dfb.png)


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

  https://github.com/w3c/manifest/pull/355

-- Commit Summary --

  * Use `https://` where possible

-- File Changes --

    M README.md (8)
    M index.html (104)

-- Patch Links --

https://github.com/w3c/manifest/pull/355.patch
https://github.com/w3c/manifest/pull/355.diff

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/manifest/pull/355