Update ssd.py

[Amit097]

Keras is updated so we should use concatenate instead of merge.

net['mbox_loc'] = merge([net['conv4_3_norm_mbox_loc_flat'], net['fc7_mbox_loc_flat'], net['conv6_2_mbox_loc_flat'], net['conv7_2_mbox_loc_flat'], net['conv8_2_mbox_loc_flat'], net['pool6_mbox_loc_flat']], mode='concat', concat_axis=1, name='mbox_loc') use this: net['mbox_loc'] = concatenate([net['conv4_3_norm_mbox_loc_flat'], net['fc7_mbox_loc_flat'], net['conv6_2_mbox_loc_flat'], net['conv7_2_mbox_loc_flat'], net['conv8_2_mbox_loc_flat'], net['pool6_mbox_loc_flat']],axis=1)

[kurianbenoy]

This PR doesn't work. Verified it.

I have asked for a reply to problem here: https://github.com/rykov8/ssd_keras/issues/149#issuecomment-503397194