The great Travis speedup

[pietroalbini]

Important: all the Travis caches needs to be cleaned before merging this. The old stable cache is broken as far as I understand, and the migration to rustfmt-preview may cause problems to the lints job. Anyone with push access to this repo can do it on this page, with the "delete all repository caches" button.

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Travis speed on this repository is pretty bad (not rustc-bad, but a simple commit takes more than 20 minutes to test), and with this PR I'm trying to solve that. There are a few problems at the moment:

1. Travis rebuilds all the dependencies two times on each build, since the build/clippy step is done with RUSTFLAGS=-Dwarnings and the test one without
2. Only the stable channel is actually tested: the beta one is missing, and the nightly one is pinned to a specific date (thanks clippy!)
3. The "lints job" uses a pinned version of rustfmt, so we need to check if it works every time we bump the pinned nightly: now that the rustfmt-preview component is available we can use it instead
4. The pinned nightly (and so rustfmt/clippy) is a bit old, so the latest versions of the tools I use locally produce different output than CI

This PR addresses those problems:

1. All the cargo calls now are done with RUSTFLAGS=-Dwarnings, so it doesn't invalidate all the cached dependencies multiple times on each build
2. I added a build job for beta and nightly (different than the one used for lints), and I allowed the nightly one to fail without failing the build (I don't want to block CI if there is a regression)
3. Switched to using rustfmt-preview on the "nightly lints" job: I'm not doing it everywhere because rustfmt stable wants different things than rustfmt nightly at the moment
4. Updated the pinned nightly and clippy to the latest working version (there are regressions affecting us in both the nightlies of clippy 0.0.188 and of clippy 0.0.189)

I have to say, the result of this is really good (we go from a 20+ minutes wait to about 3 minutes):

	Old builds	New builds without cache	New builds
stable	18 min	9 min	3 min
beta	[not tested]	9 min	3 min
nightly	[not tested]	9 min	3 min
nightly lints	22 min	21 min	3 min

[pietroalbini]

This PR is now ready to be reviewed/merged. cc @aidanhs

[aidanhs]

All caches cleared - seems ok, let's give it a shot!