Use MSVC version of the Rust compiler on AppVeyor

[micbou]

In PR #268, we use the gnu version of the Rust compiler on AppVeyor because the MSVC version does not support the new windows 10 SDK. This is fixed in the development branch of Rust but it will take some time to land in the stable version. According to Rust release process, the fix should be available in the 1.7 version (in about two months).

Meanwhile, we cannot apply the vcvars workaround because it sets the Omnisharp build configuration to 64-bit release mode, resulting in a failed build.

Our plan is to use the gnu version on AppVeyor until the stable release contains the fix. We will then replace it with the MSVC version.

[Valloric]

Thanks for filing this!