add Travis build matrix to do parallel builds for (make, CMake) x (with CUDA, without CUDA)

With so many different build options -- use make or CMake, build with CUDA, build/test without CUDA -- doing them all serially with one Travis worker is starting to take a pretty long time (30+ minutes each, even at non-peak hours with good CUDA download speeds).  This adds a build matrix to separate them into four builds, each taking < 15 minutes.

@akosiorek do you have any thoughts on why the CMake build tends to take significantly longer? I noticed that some files were built multiple times, e.g. test_caffe_main.o is built for every test rather than just once, but I'm not sure if that's significant.