Splits prediction error plots from residuals

[rebeccabilbro]

This PR closes #1031, splitting prediction error plots from residuals.

I have made the following changes:

1. Created new files, yellowbrick/regressor/prediction_error.py and tests/test_regressor/test_prediction_error.py and migrated code there from yellowbrick/regressor/residuals.py and tests/test_regressor/test_residuals.py, respectively.
2. Relocated the relevant baseline images into a new directory, tests/baseline_images/test_regressor/test_prediction_error
3. Updated yellowbrick/regressor/__init__.py
4. Updated the docs accordingly

CHECKLIST

• Is the commit message formatted correctly?
• Have you noted the new functionality/bugfix in the release notes of the next release?
• Included a sample plot to visually illustrate your changes?
• Do all of your functions and methods have docstrings?
• Have you added/updated unit tests where appropriate?
• Have you updated the baseline images if necessary?
• Have you run the unit tests using pytest?
• Is your code style correct (are you using PEP8, pyflakes)?
• Have you documented your new feature/functionality in the docs?
• Have you built the docs using make html?

[Kautumn06]

This looks great @rebeccabilbro! I was able to build the docs and checked to make sure the images generated correctly. I've updated the branch and once the tests have passed, I can merge it in. Thanks again!

[rebeccabilbro]

Thanks for the review @Kautumn06! Something weird must have been up with Travis's Miniconda image because the build has been failing the last couple weeks (even though the Appveyor Miniconda was ok). But it looks like they must have sorted it out on their end, so now the tests pass as expected! 🎉

[Kautumn06]

Thank you @rebeccabilbro! I tried to figure out the issue with the Minicoda image last weekend, but admittedly testing is not my strong suit. I was at a loss about what the problem could be, so on Tuesday I actually just went on Travis CI and tried the only thing I could think of — which was to rerun the tests and hope for the best 🙏

I'm glad that the tests are now passing as expected and thank you for merging it in!