Make the first example in Keras' Sequential Model guide pass

[cesarsouza]

The first example in Keras' sequential model guide is:

from keras.models import Sequential
from keras.layers import Dense, Activation

model = Sequential([
    Dense(32, input_shape=(784,)),
    Activation('relu'),
    Dense(10),
    Activation('softmax'),
])

and the unit test is located at https://github.com/cesarsouza/keras-sharp/blob/master/Tests/SequentialTest.cs#L44