Pin nightly version to a latest one known to be working

[Xanewok]

Lightweight version of #131 - it'd be good to start with a good working base and incrementally work from there.

r? @Manishearth

[Xanewok]

It's worth noting that I used the same cargo bisect-rustc tool from #131 but with test script rather than check - there were some changes wrt Eq and StructuralEq that needed addressing in nightly-2020-06-10 suggested there; atm this version (nightly-2020-06-01) is what builds and passes current tests.

[Manishearth]

Hmm, CI still fails, but r=me if you can get it to pass

[Xanewok]

That's because it tests using the newest nightly with rustup-toolchain-install-master: https://github.com/rust-dev-tools/rust-semverver/blob/0bbdabfc883a4e5c4d6dce21800b01649f0a70fa/ci/run.sh#L10-L21

If you don't mind, I'd prefer to change the CI script to just cargo test (that accounts for rust-toolchain) and then re-introduce the daily CI check using rustup-toolchain-install-master later at some point

[Xanewok]

CI is green now, merging as per above