Pin rustc to a date with known snapshots

I don't really expect all builders of cargo to be forced to use this rustc, and
I expect this to update frequently, but it will allow cargo PRs to land without
forcing a "put out the fires" fix for all nightly incompatibilities all at once.
This will allow rust upgrades to land separately and be reviewed separately.
auto merge of #859 : alexcrichton/cargo/issue-856, r=brson

I don't really expect all builders of cargo to be forced to use this rustc, and
I expect this to update frequently, but it will allow cargo PRs to land without
forcing a "put out the fires" fix for all nightly incompatibilities all at once.
This will allow rust upgrades to land separately and be reviewed separately.

Closes #856