Add the right version of nightly to the documentation

[chillcaw]

Since the latest versions of nightly are breaking rocket due to the ring library drama, I was wondering if we could add: ~/.cargo/bin/rustup override set nightly-2017-12-18 for people who are new to rust? (2017-12-18 is the minimum requirement for pear-codegen and therefore rocket-codegen)

[chillcaw]

This is in response to #513

[SergioBenitez]

Given the uncertainty brought on by #513, it seems prudent to specify which version of nightly to use in the documentation. This is something I was really hoping to avoid during Rocket's life as a nightly library by always building on the latest nightly. It's a real shame that an unrelated library's unwillingness to do what's best for the community has temporarily grounded Rocket.

My thoughts are that we should indicate the proper nightly version in the getting started guide as well as the README, perhaps in the "building" section. Are there additional places we should record this?

[chillcaw]

I had a brief look over your PR's for the ring library, I don't understand the unwillingness to cooperate on ring's behalf, especially in this case as you have already gone to the trouble fixing ring 0.11 for the latest nightly build.

Yes I think this is a good idea. I think definitely in the 'Getting Started' guide as rustup can be a bit of a pain for even intermediate rust users. The 'Getting Started' and the README seem to be the only places I can think of. I think listing the minimum requirement of nightly as well as the latest currently supported version of nightly for rocket (which I believe is nightly-2017-12-21) would be good too.

[SergioBenitez]

Changes made and deployed!

[hasanOryx]

I'd like to suggest looking into the option of building Rocket without dependencies, aware this may need more effort, but believe will help not sticking in such situation again, Total.js framework is doing like this.

[chillcaw]

It looks good to me! Hopefully when Rocket is ready for 0.4 we'll be able to support the newest nightlies again!