Hover Preview: Ignore w3c gradient syntax not yet supported in Chrome 25

Until Brackets is upgraded to Chrome 26, Hover Preview needs to:

1. Ignore gradient with "to" keyword

<del>2. Ignore gradients with new color stop syntax (i.e. `<color> <length>`)</del> Misguided theory.

Note that some Hover Preview unit tests <del>will need to be</del> were updated for this change.

This mitigates #3458