common rust configuration features are not supported

[dbaron]

The configuration recommended by both Travis's own documentation and by the rust documentation:

language: rust
rust:
  - stable
  - beta
  - nightly
matrix:
  allow_failures:
    - rust: nightly

is rejected by both travis lint and by the Travis WebLint, which I believe are both based on this code.

Another common piece of rust configuration (recommended by this blog post on running rustfmt in Travis CI):

cache: cargo

is also rejected. travis lint reports:

Warnings for .travis.yml:
[x] in matrix.allow_failures section: unexpected key rust, dropping
[x] unexpected key rust, dropping
[x] value for cache section is empty, dropping
[x] in cache section: unexpected key cargo, dropping