Travis: Add optional job to forecast cargo update failures

[JohanLorenzo]

For reference:

• Example of failed job

Fixes #409. Depends on #411

[coveralls]

Coverage remained the same at 39.03% when pulling 7bb3794 on JohanLorenzo:forecast-cargo-update into 4036f9e on fxbox:master.

[JohanLorenzo]

I wish we could build against the pinned version of nightly. But we can allow failures only on groups of jobs.

[coveralls]

Coverage remained the same at 38.441% when pulling 7a171dd on JohanLorenzo:forecast-cargo-update into b08ff2c on fxbox:master.

[coveralls]

Coverage remained the same at 38.441% when pulling 21cf1c2 on JohanLorenzo:forecast-cargo-update into ede7e83 on fxbox:master.

[JohanLorenzo]

r? @fabricedesre

[fabricedesre]

r=me