Bug#804612: Requires upload of bazel first

[Ghislain Vaillant <g.vaillant@imperial.ac.uk>]

According to the installation instructions [1], bazel is required to 
build TensorFlow. There is an ITP currently filed for bazel [2].

[1] 
https://github.com/tensorflow/tensorflow/blob/master/tensorflow/g3doc/get_started/os_setup.md
[2] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=782654

On a separate note, this package would be a great candidate for 
maintenance by the Debian Science Team. I might give it a go if bazel 
gets uploaded eventually.

Ghis