Auto merge of #555 - alexcrichton:retry-appveyor, r=alexcrichton
Auto merge of #555 - alexcrichton:retry-appveyor, r=alexcrichton

Surround rustc download in appveyor-retry

anatol/libc,rust-lang-nursery/libc,cactorium/libc,mneumann/libc,mneumann/libc,rust-lang-nursery/libc,cactorium/libc,rust-lang/libc,rust-lang/libc,anatol/libc,rust-lang-nursery/libc,mneumann/libc,rust-lang/libc,cactorium/libc,anatol/libc