Update serde to 0.7

[kyeah]

Updates serde to 0.7. As noted in #24, this is a breaking change :(

[dariusc93]

Strange.. When I apply the patches to the local repo i have of bson-rs i get the following when compiling an example made with the sample code on #24

expected `core::option::Option<bson::ordered::OrderedDocument>`,
    found `core::option::Option<bson::ordered::OrderedDocument>`
(expected struct `bson::ordered::OrderedDocument`,
    found a different struct `bson::ordered::OrderedDocument`)

There are also other related errors. I will run a split test from my own workspace to see if its my code or this pr.

Update: It looks like it is just my code and mongodb since mongo uses a different version of bson. Hopefully when this is pulled that mongodb can also update

[zonyitoo]

I didn't use that in my own project. So I am Ok with this change.

Is it ready to be merged?

[dariusc93]

It's good so far for me.

[kyeah]

Yeah, this is ready for merging! Once this is in, I'll make the required changes on the mongo driver.