servo: Merge #10528 - Invalidate appveyor cache when rust or cargo rev (from larsbergstrom:invalidate_cache); r=edunham

r? @edunham

AppVeyor builds are currently failing due to exceeding the cache size limits (https://ci.appveyor.com/project/servo/servo/history). This happened after the rustup. This patch should ensure we blow away the cache if the rust or cargo versions change.

See:
http://help.appveyor.com/discussions/questions/1788-multiple-build-cache-dependencies

Source-Repo: https://github.com/servo/servo
Source-Revision: ac062931e241d227d91da7e869608308b1172d4e

servo/appveyor.yml

@@ -15,8 +15,8 @@ platform:
   - x64
 
 cache:
-  - .servo
-  - .cargo
+  - .servo -> rust-nightly-date, cargo-nightly-build
+  - .cargo -> rust-nightly-date, cargo-nightly-build
 
 install:
     # Check if commit in auto branch exists in master, if exists build will be canceled.