Make serde optional

[pizzaiter]

Closes #264

This PR makes serde support optional. At the moment it is disabled by default.

I am not very sure about my changes to .travis.yml but it seems to work.

---

This change is 

[kvark]

should be 1.23.0 now

[nical]

Thanks for doing this! Since this is a breaking change, could you also bump euclid's version to 0.17.0 (so that I don't forget and break all of servo next time I publish a minor change)?
And as kvark said, now is a good time to use rustc 1.23.0 since it's the version we currently use for gecko and servo's CI.

[kvark]

@nical I wonder if this would still be considered a breaking change if serde feature is enabled by default.

[nical]

I suppose it would not be a breaking change but disabling default features is a real pain in the lower back, because each crate in the same program depending on euclid would need to separately disable default features (and if euclid one day gets several default feaures I think that you can't specifically disable a default feature, you need to disable them all and reenable the ones you wanted to keep).

So now might be a good time to just go ahead with the various breaking changes that have been withheld to avoid bumping euclid all over servo.

[kvark]

Yes, sure. I'm all for having it off by default, but in the new version. We can still patch the current one with having it on by default.

[nical]

Sure thing, although unless there are non-breaking changes that we want to make now, since I believe having serde as a default feature isn't super useful, I wouldn't bother with it and just go ahead with a round of euclid bumps in servo.

[pizzaiter]

I pushed two new commits bumping the rust version and euclid's version.

[nical]

Great, thanks!

@bors-servo r+

[bors-servo]

📌 Commit 977e136 has been approved by nical

[bors-servo]

⌛ Testing commit 977e136 with merge 7bb2617...

[bors-servo]

☀️ Test successful - status-travis
Approved by: nical
Pushing 7bb2617 to master...