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