Fix appveyor build

Closes #43.

appveyor.yml

@@ -5,13 +5,16 @@ environment:
     - TARGET: i686-pc-windows-gnu
       MSYS_BITS: 32
     - TARGET: x86_64-pc-windows-msvc
+      VCVARS: "C:\\Program Files (x86)\\Microsoft Visual Studio 14.0\\VC\\bin\\amd64\\vcvars64.bat"
     - TARGET: i686-pc-windows-msvc
+      VCVARS: "C:\\Program Files (x86)\\Microsoft Visual Studio 14.0\\VC\\bin\\vcvars32.bat"
 
 install:
   - ps: Start-FileDownload "https://static.rust-lang.org/dist/rust-nightly-${env:TARGET}.exe"
   - rust-nightly-%TARGET%.exe /VERYSILENT /NORESTART /DIR="C:\Program Files (x86)\Rust"
-  - SET PATH=%PATH%;C:\Program Files (x86)\Rust\bin
+  - set PATH=%PATH%;C:\Program Files (x86)\Rust\bin
   - if defined MSYS_BITS set PATH=%PATH%;C:\msys64\mingw%MSYS_BITS%\bin
+  - if defined VCVARS call "%VCVARS%"
   - rustc -V
   - cargo -V