data: readability.css: Fix a bit to please jigsaw.w3.org/css-validator

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>
data: readability.css: Get rid of vendor-specific property for transition.

The `transition` property is supported since Firefox 16, according to MDN.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>