Use serde 0.8

[SuperFluffy]

I updated bigint, complex, and rational to use serde 0.8, and also fixed deserialization and the serde feature as such in the rational crate (didn't add any tests, but it compiles now).

Similar to #196 for num/complex, “use serde;” needed to be removed in num/rational.

[cuviper]

The rational fixes are welcome right away. The serde upgrade changes our public interface though, so that's Yet Another Breaking Change. We should do that eventually, but let's separate this.

[cuviper]

Hmm, since it didn't require any code change, maybe our serde dependency could use an explicit range. The caret requirement uses strict semver, but we might allow something like >=0.7, <0.9. Then we should be compatible with any user's 0.7.* or 0.8.*. What do you think?

[cuviper]

We really should have at least a build test too. Can you add a line to .travis/test_nightly.sh?
(Only nightly just because I know serde won't work with our rust-1.0.0 build.)

[SuperFluffy]

Alright, the PR was split into two, see #223. I am still basing the serde update off of the other one, since building the entire num crate with the serde feature is broken otherwise.

[cuviper]

I confirmed that the dual-serde works locally, but could you add another test for this? The existing test you added will grab the latest possible, so that will be some 0.8.*. After that, you can force a downgrade like cargo update -p serde --precise 0.7.0 and build again.

[SuperFluffy]

All done, thanks for the extra requests. I finally had a reason to mess around with travis a bit. :-)

[cuviper]

Great, thanks!

@homu r+ 4f6f7b3

[homu]

⚡ Test exempted - status

[SuperFluffy]

What is the process of getting a new version of this (0.1.35?) to crates.io?

[cuviper]

I just need to go through and publish each updated crate. I'll try to do that tonight.

[cuviper]

OK, 0.1.35 is out, enjoy!