telepathy-gabble: 0.18.3 -> 0.18.4

Semi-automatic update generated by https://github.com/ryantm/nix-update tools.

This update was made based on information from https://repology.org/metapackage/telepathy-gabble/versions.

These checks were done:

- built on NixOS
- Warning: no binary found that responded to help or version flags. (This warning appears even if the package isn't expected to have binaries.)
- found 0.18.4 with grep in /nix/store/pg936ixgiw96xqsrdzbwc1civylmy1q5-telepathy-gabble-0.18.4
- found 0.18.4 in filename of file in /nix/store/pg936ixgiw96xqsrdzbwc1civylmy1q5-telepathy-gabble-0.18.4
- directory tree listing: https://gist.github.com/92190024cdfe17a3e79730f988d904f6