Use stable version of rustfmt/clippy

We currently use ruftfmt and clippy with a toolchain nightly-2019-05-17.
But the stable version have caught up to our last nightly version.

nightly nightly-05-17:
rustfmt 1.2.2-nightly (5274b49 2019-04-24)
clippy 0.0.212 (ad3269c 2019-05-14)

stable 1.36:
rustfmt 1.2.2-stable (5274b49 2019-04-24)
clippy 0.0.212 (082cfa7 2019-06-07)

We might update our nightly toolchain version, or just stick with the stable version.
I think the latter is better. It works well with the existing ecosystem (eg. IntelliJ) and requires less attention to keep up with recent version.