Mark the extension as stable.
Bump the version to 0.1.3.

Seems that the v0.1.2 tag has been pushed before releasing 0.1.2 on PGXN and
updating relevnant fields in META.json and the control file.

Rectify this by bumping to 0.1.3 and tagging as v0.1.3.

Again, just like in the 0.1.1 -> 0.1.2 bump, an empty upgrade script is
provided.