not just simple, i think it's simplest...囧...
If it cost too mach time to download the basic document, you can put "vgg16_weights_tf_dim_ordering_tf_kernels.h5" into "~/.keras/models". And then restart your computer!!
"vgg16_weights_tf_dim_ordering_tf_kernels.h5" : https://pan.baidu.com/s/1snkbQkH