Updating Travis-CI build to compile OpenCV 2.4.x
Updating Travis-CI build to compile OpenCV 2.4.x

Otherwise the Ubuntu 14 VM used by Travis-CI installs OpenCV 2.3.x
through the system package manager. This method will enable us to
switch to OpenCV 3.x later.

jshoyer/plantcv,stiphyMT/plantcv,stiphyMT/plantcv,stiphyMT/plantcv,danforthcenter/plantcv,AntonSax/plantcv,danforthcenter/plantcv,AntonSax/plantcv,jshoyer/plantcv,danforthcenter/plantcv