rebeccapurple to be added as new colour alias

[AutomatedTester]

This is being done in Gecko and might as well do it in Servo too. Gecko bug is https://bugzilla.mozilla.org/show_bug.cgi?id=1024642.

I will do the bug

[SimonSapin]

The change in color.rs should be fairly obvious, but you’ll also need to change the Servo repository to have the git submodule point to the new rust-cssparser commit.

[Manishearth]

@SimonSapin I'm working on a rust upgrade now (Linux finished, need a volunteer with a mac, also need to squash some warnings), so we can update the pointer when that happens.