Add explicit XGBoostRegressor objective to avoid deprecation error

[thunfischtoast]

What does this PR do?

Set the objective for XGBoostRegressors explicitly to reg:squarederror to prevent deprecation warnings.

Where should the reviewer start?

tpot\tpot\config\regressor.py and tpot\tpot\config\regressor_sparse.py

How should this PR be tested?

From my point of view it it not needed.

Any background context you want to provide?

What are the relevant issues?

Currently when including XGBoostRegressors in the pipeline the logs are spammed with deprecation warnings.

Related partially to problems mentioned in #877 .

Screenshots (if appropriate)

Questions:

• Do the docs need to be updated? No
• Does this PR add new (Python) dependencies? No

[coveralls]

Coverage remained the same at 96.245% when pulling f068323 on thunfischtoast:xgboost_objective_fix into 3e2a692 on EpistasisLab:development.

[weixuanfu]

Good catch! Thank you!