Fix activation functions

src/layers/activation.jl

@@ -1,12 +1,12 @@
 export σ, relu, softmax, flatten
 
+# Sigmoid
 σ(x) = 1 ./ (1 + exp.(-x))
+back!(::typeof(σ), Δ, x) = Δ .* σ(x).*(1.-σ(x))
 
-back!(::typeof(σ), Δ, x) = Δ .* σ(x)./(1.-σ(x))
-
+# Rectified Linear Unit
 relu(x) = max(0, x)
-
-back!(::typeof(relu), Δ, x) = Δ .* (x .< 0)
+back!(::typeof(relu), Δ, x) = Δ .* (x .> 0)
 
 # TODO: correct behaviour with batches
 softmax(xs) = exp.(xs) ./ sum(exp.(xs))