TF Build Matrix added for Travis

[DEKHTIARJonathan]

This PR aims to insure that TL is compatible with not only the latest version of TF.
I have taken a history of 1 year into account.

To insure that no-one is using TL with a TF version lower than 1.6.0 I have added a small test on import, it raises the following error if the TF version is inferior to 1.6.0.

>>> import tensorflow as tf
>>> import tensorlayer as tl

Traceback (most recent call last):
ImportError: __init__.py : Could not import TensorLayer.
Error: TensorLayer does not support Tensorflow version older than 1.6.0.
Please update Tensorflow with:
 - `pip install --upgrade tensorflow`
 - `pip install --upgrade tensorflow-gpu`

Highlighted Failures:

• Tensorflow == 1.5.1 : Failure => Build Job
• Tensorflow == 1.5.0 : Failure => Build Job
• Tensorflow == 1.4.1 : Failure => Build Job
• Tensorflow == 1.4.0 : Failure => Build Job
• Tensorflow == 1.3.0 : Failure => Build Job
• Tensorflow == 1.2.1 : Failure => Build Job

Travis is now building the following: Travis Build

[DEKHTIARJonathan]

@zsdonghao @lgarithm @luomai

As you can see, we have broken support for any TF version older than 1.6.0. So TL users must use a TF version which is less than 2 months old.

Is it something that we want to fix ? If not, I'll remove the build support for any version older than 1.6.0.
If we want to fix this issue, where do you want to stop backward support ?

When TF versions older than 1.6.0 are removed, we obtain the following: