Packaging dependencies

A couple of missing build- and run-time dependencies I noticed when building stb-tester from scratch on a new Ubuntu 14.04 system.

Note that the debian packaging scripts disable running the selftests so they aren't actually run as part of the build, but this gets us closer to re-enabling them.

@wmanley Do you think we should install tesseract-ocr-eng as a run-time dependency, given that the default language for `stbt.ocr` is english?