Appveyor: Source vcvarsall to get dumpbin in path

Author: mattico

appveyor.yml

@@ -17,6 +17,8 @@ test_script:
   - cargo build --release
   - cargo test
   - cargo test --release
+  - if %platform% == x64 CALL "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat" amd64
+  - if %platform% == x86 CALL "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat" amd64_x86
   - dumpbin /disasm target/debug/librustc_builtins.rlib
   - dumpbin /disasm target/release/librustc_builtins.rlib