Fixes broken build caused by pull request #132.

To make it easier to review, here is a rundown of the changes in this pull request:

• Fixes 3 vows failing with UnicodeEncodeError by calling encode('utf-8') in FileStorage's path_on_filesystem method. (Not sure if this will have unintended side effects. It would be nice if someone could pay special attention to this).
• Installs opencv from ubuntu packages. Building from source was taking too long and Travis builds were timing out.
• Removes deprecated import in app.py.
• Adds missing python-magic dependency.