Futher optimize `tile.png` and `tile-wide.png`

 * `tile.png`:

    before:        13743 B
    after:          3495 B
    ──────────────────────
    reduction:     10248 B [74.56%]

 * `tile-wide.png`

    before:         6774 B
    after:          1864 B
    ──────────────────────
    reduction:      4910 B [72.48%]

Close h5bp/html5-boilerplate#1604.
Update `apple-touch-icon-precomposed.png` and docs

 * Update the documentation regarding the Apple touch icons, by
   reflecting the recent changes brought by the release of iOS 8 and the
   new iPhone 6 and 6 Plus.

   Note: The changes do not follow all Apple's iOS Human Interface
   Guidelines¹, as in practice, the images used by the different devices
   are different² from the ones specified in the Apple documentation.

 * Update the `apple-touch-icon-precomposed.png` to be `180×180px` (this
   is now the largest image size used by any Apple device running iOS).

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¹ https://developer.apple.com/library/ios/documentation/UserExperience/Conceptual/MobileHIG/IconMatrix.html
² h5bp/html5-boilerplate#1599

Close h5bp/html5-boilerplate#1599.
Docs: Use `https://` where possible
Update `devDependencies`
Update `devDependencies`
Update to normalize.css v3.0.2

Close h5bp/html5-boilerplate#1609
Docs: Make minor correction regarding touch icons

Clarify that the iPad and iPad mini with @1x display running iOS 8 are
also using the `76×76px` touch icon.

Ref: h5bp/html5-boilerplate#1599.
Change the outdated browser prompt classname

While the outdated browser prompt currently links to `browsehappy.com`,
users are free and even encouraged to substitute the link with their own
page or site of preference.

This commit changes the `browsehappy` specific classname with the more
agnostic `browserupgrade` in order to make the link substitution
easier, but more importantly to hint to the users that they don't have
to stick with the currently default provided `browsehappy.com` link.

Ref: h5bp/html5-boilerplate#1582
     h5bp/html5-boilerplate#1608

Close h5bp/html5-boilerplate#1608
Docs: Make minor update regarding touch icons
Docs: Improve info regarding IE's document modes

Ref h5bp/html5-boilerplate#1554
Close h5bp/html5-boilerplate#1446
Add vertical centering for `iframe`

Remove gap between the `iframe` elements and the bottom
of their containers by vertically centering them.

Close h5bp/html5-boilerplate#1613
Move `update_dist_directory.sh` into `bin/travis`

Make it even more clear that the `update_dist_directory.sh`
is intended for Travis¹, and not for the user to run it.

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¹ http://docs.travis-ci.com/
Docs: Fix wrong homophone in `html.md`

Close h5bp/html5-boilerplate#1616
Update content from the `dist` directory [skip ci]