extensions: Fix shell versioning scheme used to check for updates

The extension website uses x.y for stable releases and x.y.z for unstable ones and the code in tweaks tries to map the full version string to this scheme. The old code checks if z ends with 0 which breaks the versioning scheme conversion for one unstable (x.y.90) and several (x.y.1, x.y.2, ...) stable releases. This is preventing tweaks from updating extensions on all affected micro releases each cycle.

The correct way of checking for stable releases is to check if the minor version number y is even.

Fixes #250 (closed)