rust-selectors doctests break appveyor

[jdm]

   Doc-tests selectors
error: could not execute process `C:\projects\servo\etc\rustdoc-with-private --test C:\projects\servo\components\selectors\lib.rs --crate-name selectors -L dependency=C:\projects\servo\target\debug\deps -L native=C:\projects\servo\target\debug\build\fontsan-f28177f7f4c42369\out/lib -L C:\projects\servo\target\debug\build\libz-sys-0598af38eaebd697\out/lib -L native=C:\projects\servo\target\debug\build\miniz-sys-07cc64aa136ffbd3\out -L native=C:\projects\servo\target\debug\build\heartbeats-simple-sys-9c724f9aec584f18\out\lib -L native=C:\projects\servo\target\debug\build\mozjs_sys-d466f8563f4f00d0\out/js/src -L native=C:\projects\servo\target\debug\build\angle-924e8f53a584ce62\out -L native=C:\projects\servo\target\debug\build\harfbuzz-sys-df808732578fd1c1\out/lib -L native=C:\projects\servo\.servo\msvc-dependencies\openssl\1.1.0e-vs2015\lib64 -L native=C:\projects\servo\target\debug\build\tinyfiledialogs-6059fab62bdecc97\out -L native=C:\projects\servo\target\debug\build\js-6742e96092446d2e\out/lib --extern fnv=C:\projects\servo\target\debug\deps\libfnv-d9f95aba0617f176.rlib --extern precomputed_hash=C:\projects\servo\target\debug\deps\libprecomputed_hash-91a063dc20f40156.rlib --extern cssparser=C:\projects\servo\target\debug\deps\libcssparser-9ac4357e8a35c6ea.rlib --extern matches=C:\projects\servo\target\debug\deps\libmatches-804ad663f217c404.rlib --extern smallvec=C:\projects\servo\target\debug\deps\libsmallvec-ebb1c22410667b9a.rlib --extern selectors=C:\projects\servo\target\debug\deps\libselectors-164e61d5936f0446.rlib --extern bitflags=C:\projects\servo\target\debug\deps\libbitflags-dca0fb2bd8a59897.rlib` (never executed)

[highfive]

cc @aneeshusa