Exclude PSR2 sniffs which throw warnings for underscore prefix.

PSR2.Methods.MethodDeclaration.Underscore and PSR2.Methods.MethodDeclaration.Underscore
are replaced by our own sniffs to avoid warnings for protected methods and properties with
underscore prefix but still have other checks done.

Currently you have to ignore all warnings to get phpcs builds to pass on travis. But that means we also miss out on other fixable issues. Since we are close to RC and removing the underscore prefixes is not feasible, I copied the relevant sniffs from PSR2 and removed the code which throw that warning for methods and properties.