Update utils.py

[vipulsmishra]

Added _robust_predict method in _LinUCB_n_TS_single class. It was showing error when saving the model instance of LinUCB class (which uses _LinUCB_n_TS_single class as the base model). Now, after adding the _robust_predict method, it is working fine.

[david-cortes]

Thanks for the PR, but this doesn't really address the underlying problem, since all of the other classes that take arbitrary classifiers still use unbound methods.

If you want to save models you need to use some serialization library that would support this, such as dill and similar.