Merge #279

279: Fix the build by avoiding `bitvec` 0.18.5. r=alistair23 a=jrvanwhy

`bitvec` 0.18.5 pulls in `radium` 0.3.0, which does not support platforms without atomics. Pulling in `bitvec` 0.18.4 brings in `radium` 0.4 instead (was there a bug in 0.4?) which restores our build.

Co-authored-by: Johnathan Van Why <jrvanwhy@google.com>