speed up Travis build by installing only the CUDA subpackages necessary for building

Instead of installing the entire "cuda" package, install only a couple subpackages (which the cuda package installs, along with many other large packages).  Some of the packages are unnecessary since they're related to documentation, examples, UI, etc.; others are (probably?) only unnecessary because we're not actually running the GPU tests (since Travis doesn't have GPUs).

This cuts down the download size from 833 MB (https://travis-ci.org/BVLC/caffe/builds/30988484) to 271 MB (https://travis-ci.org/jeffdonahue/caffe/builds/31008638).  It's hard to say exactly how much of an improvement this is because there seems to be a lot of variance in the CUDA download speed, but I think the download was a significant bottleneck, and 833 MB -> 271 MB is an improvement regardless of how fast the download is.