add debian:unstable test to .travis.yml

[k-okada]

c.f. #97 (comment)

[jspricke]

I'm not sure this will work as is. I was rather talking about using the packages in Debian, i.e., install all build dependencies listed here:
https://packages.debian.org/source/unstable/ros-opencv-apps
and run the build (without using the OSRF repos).

[jspricke]

So basic idea would be:

apt build-dep ros-opencv-apps
cd opencv_apps (from git)
mkdir build
cd build
cmake ..
make
``