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)

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¹ The script (`bin/update_dist_directory.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/

Close h5bp/html5-boilerplate#1593