Change TravisCI build to use new container-based infrastructure

[lukeyeager]

See docs here:
http://docs.travis-ci.com/user/migrating-from-legacy/

Places we're currently using sudo:

• https://github.com/NVIDIA/DIGITS/blob/v2.0.0-rc3/.travis.yml#L24
• https://github.com/NVIDIA/DIGITS/blob/v2.0.0-rc3/scripts/travis/install-caffe.sh#L22-L24
• And this whole file: https://github.com/NVIDIA/caffe/blob/v0.12.2/scripts/travis/travis_install.sh

That last file is the main problem. We need to make sure all the packages we need are whitelisted and then do this:
http://docs.travis-ci.com/user/migrating-from-legacy/#How-do-I-install-APT-sources-and-packages%3F

[lukeyeager]

Also, they're currently working on 14.04 support. That would greatly simplify the build process since we could get more dependencies through apt-get.

http://next.travis-ci.com/

[lukeyeager]

It's possible that upgrading to 14.04 could help with some of the performance issues we're seeing on TravisCI - #332, #357, #372.

[lukeyeager]

Another possible improvement - cache the Caffe and Torch builds on the Travis build machines.

Example of caching dependencies and specific folders:
https://github.com/NVIDIA/caffe/blob/v0.14.0-beta/.travis.yml#L17-L23

[lukeyeager]

TravisCI is going to solve this for us automatically in the next few weeks.

Starting in the first week of December 2015 (next week!) we'll begin the process of migrating the entire workload off our OpenVZ platform and onto out new GCE setup.
https://blog.travis-ci.com/2015-11-27-moving-to-a-more-elastic-future

Closing. I may address some of these other comments in future PRs.