Determine minimal supported rust version

[bmwill]

#4 identified that the use of the matches! macro pushed the msrv to 1.42. Once that PR is landed determine what the new msrv is (or should be) and add a check in CI.

[weiznich]

For diesel we use this ci snippet to verify that we do not accidentally bump our minimal supported rust version.
Generally speaking: It's just fine to choose whatever version you want there. It's just that the fix was easy and that would allow us to keep supporting that version with diesel 😉

[bmwill]

Current minimum supported rust version is 1.36.0 as of 6453d35.