Maintain TensorLayer 1.7.4

This release includes the following:

• New Update
  • Distributed Training Inception V3 : Modified optimizer and learning rate to match the paper b8b3d9c (by @jorgemf )
  • Travis test (by @jorgemf )
  • Format core library using yapf 92c861a (by @luomai )
  • Use stack_bidirectional_dynamic_rnn for multi-layers BiDynamicRNN 36ff5f9 (by @matthew-z)
  • tl.vis.save_image supports grey scale image cbbd0b7 （by @zsdonghao)
  • Update layer API for issue #289 with backlink to documentation 4866017 (by @DEKHTIARJonathan)