Address {ggplot2} 3.3.0 update

[echasnovski]

This PR deals with Travis build fails. The issue was a change in "outline.type" argument of geom_area() (see {ggplot2}'s 'NEWS.md') which affected shade_p_value() {vdiffr} test reference outputs. This PR updates those images directly (with vdiffr::manage_cases()) since there is no visible changes introduced. Note that in order to get clean tests locally, {ggplot2} version should be updated to 3.3.0.

Also:

• {ggplot2} 3.3.0 introduced warnings in stat_function() which affected visualize(method = "theoretical") cases. Seems like it wasn't a necessary change (see this {ggplot2} issue). To remove these warnings, stat_function() was replaced to an equivalent geom_path(stat = "function").
• I believe that {ggplot2} update should break {infer} CRAN checks pretty soon, so we should prepare for a soon {infer} CRAN update.

[echasnovski]

Oh, I forgot that {vdiffr} tests are disabled on CRAN, so no breaks will be there.
Nevertheless, I'll merge this because of the warnings (which are a compelling reason for a soon CRAN release).
@ismayc, @andrewpbray, @simonpcouch, please keep in mind that {ggplot2} 3.3.0 is now needed for passing tests.

[github-actions]

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