fix(ripple): Provide fallbacks for all custom properties

[BBosman]

From #361:

• Ensure all var() lookups have a second parameter
• Ensure all var() lookups have a fallback of some sort for browsers that don't support custom properties
• Run material-components-web generated CSS through postcss-custom-properties. Remove any errors and attempt to reduce warnings as much as possible, while taking into account the limitations of the tool.

While running it through postcss-custom-properties I fixed all warnings except for the Custom property ignored: not scoped to the top-level :root element ones.

Passing warnings: false to postcss-custom-properties suppresses that specific warning though, resulting in a warning free build.

[traviskaufman]

LGTM! Thanks for doing this!

@sgomes can we close #361 once this is merged?