patch Mocha.jl mnist.jl example to use cpu backend

gistfile1.diff

index dd2a40f..98458e3 100644
--- a/examples/mnist/mnist.jl
+++ b/examples/mnist/mnist.jl
@@ -1,7 +1,7 @@
 #ENV["MOCHA_USE_NATIVE_EXT"] = "true"
 #ENV["OMP_NUM_THREADS"] = 1
 #blas_set_num_threads(1)
-ENV["MOCHA_USE_CUDA"] = "true"
+#ENV["MOCHA_USE_CUDA"] = "true"

 using Mocha
 srand(12345678)
@@ -15,7 +15,7 @@ fc1_layer   = InnerProductLayer(name="ip1", output_dim=500, neuron=Neurons.ReLU(
 fc2_layer   = InnerProductLayer(name="ip2", output_dim=10, bottoms=[:ip1], tops=[:ip2])
 loss_layer  = SoftmaxLossLayer(name="loss", bottoms=[:ip2,:label])

-backend = GPUBackend()
+backend = CPUBackend()
 init(backend)

 common_layers = [conv_layer, pool_layer, conv2_layer, pool2_layer, fc1_layer, fc2_layer]