Make Travis automatically update the server branch

Configure Travis to automatically update the content from
the branch¹ intended for the server by executing the
`update_server_branch.sh` script whenever the tests pass.

Note: The `update_server_branch.sh` script will only update
the content if the changes were made in the `master` branch.

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¹ https://github.com/h5bp/html5boilerplate.com/wiki