Change neptune.ml to neptune.ai (krzjoa#31)

Neptune.ai is the correct address now. Thanks!

Author: PawNep

README.md

@@ -416,7 +416,7 @@
 * [Xcessiv](https://github.com/reiinakano/xcessiv) - A web-based application for quick, scalable, and automated hyperparameter tuning and stacked ensembling.
 * [Persimmon](https://github.com/AlvarBer/Persimmon) - A visual dataflow programming language for sklearn.
 * [Ax](https://github.com/facebook/Ax) - Adaptive Experimentation Platform. <img height="20" src="img/sklearn_big.png" alt="sklearn">
-* [Neptune](https://neptune.ml) - A lightweight ML experiment tracking, results visualization and management tool.
+* [Neptune](https://neptune.ai) - A lightweight ML experiment tracking, results visualization and management tool.
 
 ## Evaluation
 * [recmetrics](https://github.com/statisticianinstilettos/recmetrics) - Library of useful metrics and plots for evaluating recommender systems.