Proper handling of versioning in docker file + travis config

[simonclausen]

There should be only one truth to what version is being built and released by travis.

Suggestion: set bind version in travis config and inject that via variable into the Docker build. Would require both version and SHA of bind source be handled in the travis config.

[simonclausen]

Will probably be done in a different way as per dnscryptio/dnscrypt-wrapper-docker#11