Make Travis automatically update `dist` directory

Configure Travis to automatically update the content from the `dist`
directory by executing a script¹ that will regenerate the content from
the `dist` directory and commit any new changes to the `master` branch.

This change will help in the development process by:

  * ensuring that the content from the `dist` directory is always
    in sync with the rest of the content

  * removing the need to execute the build step locally everytime
    a change is made (especially in the case of trivial changes,
    such as, typos)

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

¹ The script (`https://github.com/h5bp-bot/scripts/blob/f37573be6500f16f9a96445e7fbfcb3f5e858156/commit_build_changes.sh`)
will only be executed if the tests pass, and will only run if the changes
are made in the `master` branch.

See also: http://docs.travis-ci.com/user/encryption-keys/