Add --keep-going flag to first make so that any targets that can be
built are built; write out full -j flag as --jobs for clarity.
Run Travis build on all branches (remove whitelist from .travis.yml).
Replace CUSTOM_CXX env var to specify non-default C++ compiler.
Add blank lines between fields for readability.
Merge pull request #694 from jeffdonahue/travis-tweaks

Travis tweaks -- make --keep-going; build on all branches; replace CUSTOM_CXX <eom>
fix bug that breaks MATLAB 2014a compilation
Merge pull request #696 from rbgirshick/matlab_2014a_fix

Fix bug that breaks MATLAB 2014a compilation
Fix SoftmaxLayerTest: forgot to change this one to use DtypesAndDevices;
was causing Travis build to randomly fail if a previous test had set the
mode to GPU (which no test that is run by 'make runtestnogpu' should, so
I guess there's another bug somewhere).
Merge pull request #698 from jeffdonahue/softmax-layer-test-devices-fix

Fix SoftmaxLayerTest: forgot to change this one to use DtypesAndDevices;
fix choice of clang++ in OS X build
configure Makefile for CPU-only build
stub out GPU layer methods to crash loudly in CPU-only mode
add guards to drop GPU code in CPU-only mode
switch travis build to CPU-only