Update appveyor script

appveyor.yml

@@ -1,14 +1,17 @@
 environment:
   matrix:
-    - RUST: 1.6.0
+    - RUST: stable
       BITS: 32
-    - RUST: 1.6.0
+    - RUST: stable
       BITS: 64
 
 install:
   - IF "%BITS%" == "32" SET ARCH=i686
   - IF "%BITS%" == "64" SET ARCH=x86_64
-  - SET RUST_URL=https://static.rust-lang.org/dist/rust-%RUST%-%ARCH%-pc-windows-gnu.exe
+  - ps: Start-FileDownload "https://static.rust-lang.org/dist/channel-rust-stable"
+  - ps: $env:RUST_VERSION = Get-Content channel-rust-stable | select -first 1 | %{$_.split('-')[1]}
+  - if NOT "%RUST%" == "stable" set RUST_VERSION=%RUST%
+  - SET RUST_URL=https://static.rust-lang.org/dist/rust-%RUST_VERSION%-%ARCH%-pc-windows-gnu.exe
   - SET PATH=C:\Rust\bin;C:\msys64\mingw%BITS%\bin;%PATH%;C:\msys64\usr\bin
   - ps: Start-FileDownload $Env:RUST_URL -FileName rust-dist.exe
   - rust-dist.exe /VERYSILENT /NORESTART /COMPONENTS="Rustc,Gcc,Cargo,Std" /DIR="C:\Rust"