`multirust update` do not fetch newest `nightly`

Multirust version:

```
multirust 0.7.0 (a20ea4a38 2016-01-06 18:09:30 -0800)
```

`multirust update` output:

```
multirust: updating existing install for 'stable'
rustup: gpg available. signatures will be verified
rustup: downloading manifest for 'stable'
rustup: downloading toolchain for 'stable'
rustup: 'stable' is already up to date
multirust: updating existing install for 'beta'
rustup: gpg available. signatures will be verified
rustup: downloading manifest for 'beta'
rustup: downloading toolchain for 'beta'
rustup: 'beta' is already up to date
multirust: updating existing install for 'nightly'
rustup: gpg available. signatures will be verified
rustup: downloading manifest for 'nightly'
rustup: downloading toolchain for 'nightly'
rustup: 'nightly' is already up to date
multirust: 'stable' update succeeded
multirust: 'beta' update succeeded
multirust: 'nightly' update succeeded
multirust: stable revision:

rustc 1.5.0 (3d7cd77e4 2015-12-04)
cargo 0.6.0-nightly (e1ed995 2015-10-22)

multirust: beta revision:

rustc 1.6.0-beta.1 (2e48b593c 2015-12-09)
cargo 0.7.0-nightly (1af03be 2015-12-08)

multirust: nightly revision:

rustc 1.7.0-nightly (d5e229057 2016-01-04)
cargo 0.8.0-nightly (08da2f5 2015-12-21)
```

But what I have seen on https://static.rust-lang.org/dist/ there already are versions for 2016-01-05 and 2016-01-08.

Start by reproducing `multirust update` with stable, beta, and nightly installed, then compare the selected nightly revision with the dated manifests listed on static.rust-lang.org/dist/. Trace the update path from the `multirust update` entry point and identify why a newer nightly is not selected. Done means the command fetches the newest available nightly and reports its updated revision.

Written by the indexing model from the issue text.