need to upgrade proc_macro

[gnzlbg]

cc @alexcrichton

CI started failing yesterday: https://ci.appveyor.com/project/rust-lang-libs/stdsimd/build/1.0.1243#L114

[alexcrichton]

Yeah this is waiting on dtolnay/syn#426 now

[lu-zero]

It got merged and #455 makes the CI a bit greener :)

[alexcrichton]

Should now be done!