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;