Auto merge of #130 - japaric:rustc-cfg-up, r=alexcrichton
Auto merge of #130 - japaric:rustc-cfg-up, r=alexcrichton

bump rustc-cfg dependency to v0.3.0

the older no longer works with recent nightlies as target_family
disappeared from `rustc --print cfg`'s output

japaric/rustc-builtins,japaric/rustc-builtins