Drop support for anything but stable, beta, (nightly)

[fiveop]

libc's CI only tests against stable, beta and nighlty (and a single build seems to run against 1.0.0 for some reason). Since we are a wrapper for libc, I would like to drop support for anything but stable, beta (and working towards getting nighlty green).

If others are not comfortable with this, I would suggest to altenatively only support the n latest releases (in addition to beta and nightly), where n has to be clearly defined.

[fiveop]

Closed in favour of #444.