Untangle curve of listing 3.4 - 3.5.

[ruddyscent]

• Change variable name, final_cost to final_coeffs. The final_cost is not cost, but coefficients.
• Sort x_train and y1 before plotting them. The tangled curve is due to the unordered x_train. I could fix it using np.argsort.
• Add seed to random for the reproducibility of the result.
• Set an environment variable to suppress debugging messages from TensorFlow.