fixed alpha setting bug in _RegressionPlotter scatter

There's a known issue in matplotlib where if alpha is explicitly set for an axis globally, the 4th column in an RGBA array passed to the color argument in plots is ignored. Because seaborn sets a default alpha value in _RegressionPlotter, the alpha is always ignored even when passed. This PR just adds a check to see if the color property in scatter_kws is 4D (i.e., RGBA), and omits setting the default if so. All tests pass.